We used to live in the same city as BM and it meant we were there to be dragged into her chaos and last minute changes and her childish attempts at breaking us up etc, etc. The time I called the police on her as she refused to leave our home and pushed me to the ground. The time when BM was allowed into our new house that I'd asked my husband she not step foot in. And then we were suddenly a storage facility for her stuff in our garage without my consultation. Plus there is always the possibility of bumping into her when out, left me kind of paranoid. 

Then the best thing for my marriage happened and we moved from the west coast to the Midwest and we went to a long distance parenting plan. The drama dropped significantly and it has been a blessing to us. I loved knowing that BM wasn't going to be stepping into my home and I wouldn't bump into when grocery shopping.

I am safe to say step life is not for you.
 The real problem is how do you respect someone who abandoned there kids. 
'There kids are they problem, not the SO problem.  There has to be kid time and adult alone time.   As it's ok to have the kids over. But there also  be a adult only child free weekend , getaway.

I think in general you're probably on to something. But in our case we lived a minimum 45-minute drive from BM (longer during rush hour) and there is no distance she would not go to cause trouble. If the mood struck her, she would literally drive to our house at all hours of the night just to make some sort of over the top scene. One time on NYE she sat in her car blocking out driveway from about 6pm until 3am to make sure we didn't go out and leave the SDs with a sitter (which we were not planning to do anyway). It was insanity.

On the flip side, XH and I live a 5 minute walk from each other and it's pretty great. I think the key here is that neither of us are unstable. We have appropriate boundaries and can behave respectfully even if we're pissed at each other. It makes life so much easier for DD. 

DH had the good sense to move within the same area, kids were young during a contentious divorce (of course contentious on BMs part). However, the travel distance in reality was 1.5 hours usually, maybe 1 hour if timing was perfect. Made elementary school drop-off a nightmare but no one could just drop by.  However, BM knew my schedule and sometimes she and DH would take advantage of my commute to ferry the girls to/from dropoff. The LAST time that happened without my ok, I was not consulted by either one of the parents and as I approached the transit center here come the SDs running up to me...WTF?  I had only my bike to get to my car after the transit drop off. Zero plans to take them anywhere. I immediately contacted DH and told him off. I informed the SDs that they had two choices either wait for me to get my car and pick them up at the transit drop off point or take the bus up to where my car was. Both safe options. Neither one was happy about this and I was honest and told them their parents did not let me know I was taking them to our home so this was the result. (I can't remember why DH couldn't pick them up but there was a reason that I didn't know about, likey a work meeting because normally he was good about picking them up back then). I informed DH that would not happen ever again without my OK - he stuck to it.

Luckily BM only was in our home 2x for birthdays. The last was at OSDs 12th or 13th. Was very awkward. When OSD started demanding her things from our home (like furniture!  Um, no.) DH told her he would have the things OK for her to get but OSD needed to arrange the pickup. BM never did.  Good.

2 hours thank god. We used to live about 10 minutes from her but SS was on eow visitation. We moved to a coastal area which made it a shorter distance for me to visit my family. She had gone to court for a document to say SS shouldn't go to her any more but when she heard we had moved she went ballistic, threatening us with lawyers because she didn't yet have our address. Took her about 2, hours to make the threat lol. 

1200 Miles worked well for us.  We met shortly after she had moved away from SpermLand after HS with her 14mo old son on her hip.  We were just over 1200 Miles from SpermLand at that point.  When we married I was living less than 700 miles from SpermLand.  That was a short term assignment before our move to more than 2000 Miles from SpermLand.  We were there for 91 days after we married.  12 years later we moved to over 2800 Miles from SpermLand.  Then for the last few months before SS turned 18, we moved again to 2300 Miles from SpermLand.  We purposely avoided ever living anywhere near SpermLand while our son was growing up.  

No regular interface with the SpermClan other than visitation related calls and periodic banshee rants from the SpermGrandHag, and a couple of court hearings.

Distance. from the X ... is a major friend to those in a blended marriage.  IMHO and experience anyway.  Most importantly, it is best for the kids involved IMHO.   Far too many Xs can't be reasonable with each other and can't avoid victimizing their kids.  Distance may not stop all of it, but it provides a major buffer that keeps the impact on the kids and both sides of the blended family equation compartmentalized mostly to just the visitation schedule since a LD CO and visitation schedule clearly establishes the CP/NCP designation, schedule, balance of power, etc.....  No one really needs to fight about things in a LD custody/visitation/support order barring any kid best interest issues.  Certainly they can fight about things, but it really has no point.  Of course if one side or the other is abusive to the Skid, etc... bring on the fight. Just make sure that the toxic side is destroyed using every legal, financial, and social weapon available. Know the CO inside out, upside down, and backwards. Know the supplemental jurisdictional rules, know the State regulations for the State where the CO is issued, as well as for the State you live in with the Skid.  We used the ability to change venu to our State as a big stick to keep them in check.  We could have had Jurisdiction transferred to TX once SS was resident there for 

For us, the conflicts were associated with protecting SS from the lies, manipulations, and PASing efforts by the SpermGrandHag. The Spermidiot was just about non existent even during visitations.  For the 16+ years we lived under the CO, SS saw his Spermidiot no more than short portions of a day or two during visitation.  There was even a 2yr period where the Spermidiot moved to the opposite coast of the US without informing DW and SS continued to do his visitations in SpermLand.  We found out about it approximately 10yrs after the fact when we moved from PA to DE.  SS nonchalantly commented "Oh. Dad (Firstname) lived here for a couple of years."  That initiated a confused discussion where it unfolded that Daddy Dickhead had taken a job, moved, and SS did not see him for more than 2yrs.  There were other periods of a year or more where they refused all visitation while crying poor mouth that they could not afford their COd half of visitation travel costs.

I advise "as far as is possible" as the ideal distance anyone should live to their X.  With  a kid in the mix, it is imperative that both sides can have as close to independent relationships with the kid and the kid with them.  I lived over 1200miles from my XW for nearly 4 years though after we met and married, DW, SS and I moved back to the city where I had met, married, and lived with my XW. I never interfaced with her during the nearly 14 years we lived there after we moved. I saw her only once. I was at a business lunch. After my party was seated in an elevated section of the restaurant, I noticed my XW seated about 30ft away with her two sons and the Geriatric Fortune 500 Executive Sugar/Baby daddy cheat buddy  she was riding while we were married. It was sad. I had married a beautiful very smart college athelete who had tragically and unbeknownst to me been a void of character serially adulterous whore.  

My XILs, I ran into a few times. Once with DW and SS at a restaurant near our home, and couple of times at restaurants when I was out for lunch during work.  I could have never layed eyes on any of them ever again and not lamented it for a microsecond.

Regarding Skids, we see so many examples, on STalk, Reddit, etc, etc, etc, where drama is maximized in situations where there is a local visitation schedule.  Though certainly there can be drama in long distance visitation schedule situations. Particularly when it is the CP who is the source of the drama and a SKid is polluted by that during NCP visitations.

To reiterate my answer to the question. 1200 Miles. Minimum.

Next town over, which is a mere 15 minutes. *crazy*

Most of the time she was about 30 minutes to an hour away and the drama was annoying.  When SS lived with her, she expected us to keep him for a couple of weeks during the summer but we both worked, so he stayed with MIL during the summer.  I remember the custody order stating every other weekend but don't remember anything about summer and holidays.  She sent him to live with us, then moved close by for a time and it was a nightmare.  Constant phone calls, attempts to entice SS to come back to live with her.  She moved 30 minutes to an hour away again and the drama died down some.  It died down pretty much completely when she moved four hours sway and I got pregnant.  There's good and bad to living closer by and further away. Once she left the state we pretty much had SS all the time.  There were no breaks except when he'd stay with MIL.  But there was no drama so it was a trade off.  

4.5 miles...10 minute drive... and she barley saw them.. just created constant chaos via cellphone.. 

I would not be married--or I'd be married to someone else--if BM and skids had lived near us. Luckily for us, BM ran off to another state with the skids and her affair BF soon after throwing DH out of the house. While she was gone, DH moved to another state where he had connections, family, and a new job. When BM and BF split, she moved back "home," which was then 3 hours from where DH was living. It was workable re: visitation--3 hr drive and really fast flight. Then BM moved 5 more hours away. All the while, the "fam" narrative is that DH abandoned all of them, I guess because he wasn't back there waiting for BM's fling to sour. 

Whatever: She is so controlling, demanding, and toxic, and there was such successful PASing and turmoil from a distance that I never would have survived more frequent contact with them. Distance saved our marriage. Now we're a nice comfy 10 hours away. 

The Skunk Ape used to live fairly close to DH (before I came along). Fortunately she was so lazy she couldn't ever be bothered to leave the couch to drop by. I guess that's where Little Idiot gets it from.

Girhippoville is in the county next door.  Always dead I will ruin into their clan.  It's happened before.