Sorry I missed your post earlier, but want to welcome you to BYC.
The diagram is referenced in Storey's Guide to Raising Chickens…Quoting from the text " If roosting space is at a premium, step-stair the perches and space them 12" apart". 12 inches is both measurements in your diagram. With this arrangement IMO you will see some competition in the flock to get the best = highest roosts. If the coveted roosts are lower, the lower in pecking order may not be able to access the other roosts. According to Storey an other respected authors providing roosting space on the same level will keep the flock happier. If you have the space, better to place the 2x4 roosts 2 feet off the floor, spaced at least 18 inches apart and 18 inches off the walls. They will still jockey for closed to the window, furthest from the chicken door, etc. But happy hens lay great eggs. My roosts are equal and the girls still fuss at roost time and shift and fuss to get the position they want.
Good luck with your decision.