So the Scale and Location which you shoot is the deciding factor. Remove one or the other. The location method will reward people with good aim, and the scale will reward people who can be patient. Pick your favorite and lean in. Waiting for both to align in a hairy situation is too complex to be usable, considering even at max damage it does less than half of his starting pistol.
The headshot only requirement is what I would prefer, but I could see the scale being good too. (Defaulting to 50% on a miss.) I like it being unique and all, but it needs to feel good not clunky for people who grind it out.