I'm planning to mount my nest box (single box, only have 2 hens) on one wall of the coop.What's the minimum it should it be off the floor, so that floor space is not lost - in other words, so the chickens can walk under it comfortably? The height of a chicken, higher/lower? I want to put it under a window, but I'm not sure there's enough room there to not lose floor space.