The 20 inches sounds good. And yes, mine always face toward the window. They may start off this way and that, but they always end up facing forward by the time they're all settled. I think if a window wasn't there, they'd probably go either way.
The edge of my roost is 10 inches from the wall...so close to 12 centered. I have breeds such as BO, BA, BR, etc., and they have no problems 'fitting.' I agree that 18 inches would most likely be enough, 20 inches would be plenty. It kind of depends on how wide your roosts are - are you...