You determine his actions by using your brain and eyes together. I think I can see if someone is running away from me. I grant you that it is a hard skill to pick up just shooting at paper targets.
If one cannot determine if someone has his back to you and running out the door - well, not much can be done about that.
How about this - you maintain the sight picture on the center of the back and if you see the person turn - you shoot him.
I will opine again that if the bad guy is clearly leaving - let him go. You do what you want.