Reasonable radius to move after a divorce?

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Right after the auction closed, I went to my college reunion. It was fantastic, and I had a ton of conversations about motherhood, careers, staying ourselves, and how our lives and friendships change. That's inspired me to start a summer email series on friendships and how what we need changes as we age and as we parent. If you get emails from me already, you'll get these. If you're not on the list yet, sign up over on the right.

Then I got a nasty head cold, and have been doing nothing except coughing and doing Flash Consults.

But here's an email asking for advice that's kind of specific to the NYC-area, so if you know, please contribute advice:

"I'm drafting our divorce agreement and want to hear about people who have divorced and share custody of their child(ren), and about how they manage the question of where to live? We have a 60/40 split (me/my son's dad), but do not have a good relationship. My son is a toddler (3.5 years) and I'm trying to think long-term. 

I'm particularly interested in NYC stories/situations. Some radius clauses can be as high as 50 miles, but that won't work here. Five miles can mean several worlds away in this city, and would have implications for school commuting and the like. 

I'm looking for something that permits flexibility but is also relatively easy on everyone. I love where I live and want to keep living here for as long as I can, but NY is so changeable and the rents only ever go up. 

Please let me know what you know. Trying to come up with a workable solution. Need some suggestions as soon as you can bring them! "

I think a lot of this depends on how you see the custody going. The more frequent switches you have, the more important it is to live closer together. If you eventually go to a week-on/week-off schedule, then distance matters less (although if one house is farther away from the child's school, that's a burden on that parent).

I might also make the constraint be time instead of distance. Something that's a straight shot on the subway can take half the time to get to that something technically the same distance away but with no straight route does. So maybe deciding what's a reasonable travel time is the constraint that you want to set, instead of flat distance.

You also have to consider whether it's important for the two of you to be in the same boro or not. You can get from Manhattan to Brooklyn or Queens faster than you can get uptown or downtown, but some people don't like being across water. Same with Hoboken or Jersey City.

Once you have some constraints, you might also want to set some kind of method for deciding if one of you wants to move out of the agreed-upon radius--either triggering visits to a mediator, or opening up reevaluation of custody balance, or something like that, so that you have the opportunity to negotiate terms of a move instead of automatically having it prohibited. (It might be ok for one parent to move out of the zone if that parent took on the burden for transportation for switches, for example, or something like that.)

Anyone else?