Hand spray rough in

I've got a shower hand spray to rough in. Trouble is it appears ill need to know the exact wall thickness, but we aren't sure what wil be used for shower tile yet. There is a flow valve near this fitting which is securely fastened, so there's very little wiggle room in the pipe where it will be mounted.

Any advice for a good looking flush install?

Put the THREADED elbow far enough back into the wall so you can use a 1/2" brass nipple between it and the wall fitting.

I have sweated copper coming out of the valve, so I'll have to sweat on a drop ear elbow. Problem is I don't know what size of nipple to use once that elbow fitting is there. And if I fasten that elbow to the framing, then it'll be REALLY firm leaving no "wiggle room" to get the piece in the picture closely mated to the wall.

Do I just need to take a stab at guessing the thickness of my finished shower wall? It will be my first shower wall so I'm nervous about that part. It'll be 1/2" cement board, kerdi membrane, and travertine.

Some fixtures (Kohler is one with some of their stuff) are not very friendly and require sometimes as little as 1/16" tolerance to fit right. If your fitting in the wall is back further, you can use a threaded brass nipple - worst case, you get one custom made, but a good plumbing supply house has a pretty good range of sizes.