Bathtub valve position

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.

Have you bought the tile yet? it can be anywhere from about 3/8" to nearly 3/4" thick or even more. The cbu is nominally 1/2" (note Hardibacker, although called 1/2" isn't - sort of like plywood these days), but the exact dimmension depends on the brand. If the tile is actually a slate lookalike tile, then the thickness is probably more consistent. If it is real stone, then you may need mediumset mortar vs thinset, so the actual thickness is subject to the consistency of the tile.

Have I confused you yet?

If tile, then add 1/2" + say 1/8" for the thinset + the thickness of the tile. The thinset may end up thicker or thinner, and the cbu may be like 7/16" vs 1/2".

You really need to measure the real things and then fudge the thickness of the mortar.

Don't forget to put a vapor barrier over the studs before you put up the cbu.

wall

There is a fair amount of leeway with that valve, so make your best guess as to what the thickness will be and set the face of the ground flange there.