The stroke will end when the stroke is over, you dont need to make it stop just because you dont want to go longer than your backstroke. But you also dont need over extend it for no reason.
What I mean is COMPLETE YOUR STROKE. IMO when your shooting off the rail there are more important things than the length of your followthrough. (Im not saying you should be jabbing at the ball)
1. Dont try to do more with the cueball than you have to, take a little tougher shot for your next shot and make sure you make this one.
2. Level cue! Elevation will make the cueball curve on off center hits.
3. Proper bridge. If your hand is on the rail so is the cue. Make a bridge where the cue is sliding on the rail and your bridge is holding it in place, not where your cue is sliding on top of your hand, as this leads to more elevation of the cue.
4. Center ball, center ball, center ball. Oh, did I say you dont want to put left or right english on the shot?
You may want to experiment with looking at the cueball instead of the objectball on this shot. I dont recommend it often, but a proper strike on the cueball is critical here so you might try it and see what works best for you.
Just some suggestions.
Woody