How deep should a Delta shower valve be installed?

According to the instructions, it says the plasterguard should be flush with the finished wall which would leave about 1" of the valve sticking out, but then they say it can be installed 3/8 deeper if necessary. But then on Delta's Youtube channel it looks as if only 1/4" is sticking out of the wall.

What is the minimum amount of the shower valve I can have sticking out of the wall and what happens if you fuck it up? My walls are thick and I can't have it flush with the plasterguard. It's the R10000 with no diverter.