I would pick out a least-drafty spot for the roost and screw up a 2x4 on its flat side, tust to keep it simple.
Figure 8-10' per bird, or a little more, and not less than 18" away from the wall, and about 30" off the floor ( larger birds can injusr legs and hips jumping off higher roosts.) If...
You hae the makings of a great coop there! You might want to cut down on the drafts through the walls though I wouldn't get really fussy about it. The main thing it needs is ventilation, as high in the coop as you can get it, so the chickens' warm, humid, ammonia-y output can eschange with the...