I hate to ask such a basic question, but....

I'm about to install my new Delta R1000 rough valve. What size wood do you guys use for the stringer? By my arithmetic, it looks like it should be about 1.5 inches from the back of the studs. Shower framing is 2x4s, 1/2" permabase, Kerdi, 1/4" thick tiles. Does my math sound right?

I know the second one of these I install will be a piece of cake. But this is the first.

Thanx!