How far apart should I space my roosting perches in our new coop?

We are in the process of retro-fitting a shed to be our "new" chicken coop. The shed is approx. 10'x10' (about 6' tall) and we have put in 6 nest boxes for the girls - we put them on the outside of the shed so we can acces them easily as well as keep the eggs free of droppings from above

My question is this: How far apart should I space the roosts? I know it needs to be at least 12" from the wall (no problem - I have plenty of space), but I need to know how far apart the roosts should be. I was thinking 20" but would like some input.