Where to mount shower valve?

I know you mount it with the plaster ground flush to the finish surface but how am I supposed to tell where that is considering the finish surface isnt there?

Ill be using 5/8 Durock and I dont know what tile yet, something under 12 inches.

Is there some sort of process to do this?

It's simply not true that the guards are always set flush to the finished surface: it depends on the valve. It's always a bad idea to set the valve
at one extreme or the other of the allowed depth: there's always some leeway, and you should use it. You can calculate the wall finish depth closely
enough, but you gotta have at least some idea if you're using 3/16 mosaic tiles or 1 1/8 stone slabs, etc.

I highly suggest you not install the valve until you've chosen the tile! While there's a range of depth where the trim will fit (and that is what the manufactuer considers acceptable) YOU may not like the way it looks if it is at one extreme or the other. If it is too far forward, you wont' be able to tighten the trim up. If it's too far back, the screws nor the handle(s) will fit. SOme manufacturers offer an extension kit, but that may then make it stick out further than you like, but it will fit. Depending on the manufacturer, some have very tight tolerances about depth.