I'm finishing a bathroom and I'm to the point of installing the valve for the bathtub/shower. I have the Moentrol valve with a plastic thing on the face that says it is supposed to be flush with the finished wall. We plan to finish the surround of the tub in tile--the kind that looks a little rustic like slate. So, my question, just how thick will all the material be once it is attached to the studs? That would be the thickness of the backer board, mortar and tile.