First, while a few brands of cbu allow 1/4" for walls, most pros would not use anything other than a nominal 1/2". Then, without knowing what brand and model, it's impossible to tell you where the rough-in valve has to be - sort of like asking how high is high, or how fast is fast. If the mud guard is still attached (they may have thrown it away), it will usually have min/max lines on it to show where the trim will fit. Note, one end or the other may not actually LOOK good to you, but it will fit. If the valve is too far in the wall, some companies make extension kits. There's no easy way to fix one that's too far out. Basically, attach the trim and see what the gap is between it and the valve, then make the wall-valve spacing the way you want it.