Nest box placement is something I've wondered about, too.. So, if 18" to 24" is average, what happens if you've got two rows of nestboxes on top of one another? My plan is basically to have two horizontal rows of four boxes...if I put the lower boxes 24" off the floor, the bottom of the second row would be about 36" off the floor.
Is that too high?
If not...would I still want to put the roosts higher than the nestboxes, if the nestboxes are 36" off the floor? I can't imagine a roost being 48" off the floor, especially considering that our coop will be, shall we say, vertically challenged as it is..(about 5' interior height).