There is no perfect answer because no matter what spacing is stated someone can always come up with a counter argument using some combination of unique engines and cars. (e.g. the MTH Coors engine was especially wide and some engines/car may have tremendous overhang). I think we all try to design to a 99% solution; not perfect but a reasonable tradeoff given the cars and engines we expect to run and the space restrictions we all have.
That all said, years ago I standardized on a minimum track separation of 4.5" for 0-72, 0-63 and 0-56 concentric curves (with 3 1/2" on straights). I don't recall ever having a problem in normal day-to-day operation since I am not running long cars or long engines on the smaller diameter tracks.
