Generally, the plaster guard has lines on it representing the range of tolerance. Or the thickness of the plaster guard itself IS the range of tolerance.
Set a wood block such that the face of the dog ear elbow will be somewhere behind the inner surface of the rough wall. If you use a threaded elbow, you have to fine tune the final length of the tub spout nipple. Hard to plan this down to the 1/16" in advance. The easiest to work with are:
> stub out copper pipe instead, and use a slip spout
> stub ou the threaded nipple, and use a Delta spout with adapter....plenty of tolerance on length with that one.