I think they usually recommend 10" of length for each chicken and to place it 18" from the wall.
I think 1' from the wall would work and still give them enough room to be comfortable. Sometimes, if you have it too close to the wall, when they poop, it hits the wall and makes a mess. That may be where the 18" from the wall recommendation comes from.
If you need to put in 2 roosts parallel to each other, then I think the second one is supposed to be 1' away.