I'm really surprised the Germans didn't come up with a simple way of judging ranges for face shots. I would have used to inverted staples, |___| for on trigger, and |______| for off trigger. When the wingspan fills the first start shooting, and when it fills the second stop and pull / roll away. Final product would have looked like this |__|___|__| (middle two uprights for on trigger, outer two for off trigger).
But are cockpit shots always made "exactly" head on, with the plane squared up, or perhaps from slight angles?