If you have room for a high roost, the bantams will use that, even if it's close to the roof...
How far from the roof should it be for the bantams 10-12 inches???
I don't know how far it should be but in my coop the conduit for lighting is nailed to the bottom of 2x8 floor joist they fly up (8 feet off the ground) and roost just fine on the conduit in between the joist, head room is only 7½" up there...
So 10-12" should be more than enough...