I use a 2x6 and push it against the back wall. Then I use four 3-1/8" screws to secure it. One big mistake is not factoring in the thickness of the backer board on the main wall. So I actually center the valve at 15-1/2" since the footprint of the shower with backer boards will be 30" x 49" - it's not a big shower. If the valve is the correct depth, the finished tile wall will be almost flush with the black plaster guard
