Originally posted by kaypy:

Rule of thumb for starting out: if you are horizontal and just close enough to get the target health bar popping up, then fire at the health bar.

This is the most directly correct answer, I think. Use the target's health bar, NOT some point on the target itself. Not foolproof, but a better place to start. You have to aim a LOT higher than I'd have thought to have a chance to hit your mark.