I'm leaving open the areas that I need access to. I ended up going with hardie board. It was more highly recommended by several sources and also feels more rigid. So, I've got .42" for hardie board, and about .5" for tile and thin set. The valve is exactly 3.5" deep (to the front edge of the plaster guard), so a support block would need to be recessed about 2.5" inches in order for the plaster guard to sit flush with the finished wall.
I'm finding a lot less on how to get the depth of the diverter valve set, and it does not have a plaster guard or any sort of guide on how deep to set it.
Last edited by swindmill; 10-26-2011 at 04:48 PM.