how far back should my back hand move do u guys think?
If you go setup to the cue ball on any shot and place your cue tip at the cue ball, your forearm to your elbow should be 90 degrees. Wherever your grip hand is at cue ball impact is where it should be with that 90 degrees.
Just remember that this is for a specific bridge length too, which should also be consistent. But if you change your bridge length, then you have to change your grip position to achieve 90 degrees at impact. Recall two of the components of Scott's equation. Bridge length and grip position. If one changes, so must the other.