I do something similar to Alistair only I fold all except my forefinger to make a 'pistol fist' with my partly extended thumb as the 'trigger' and forefinger as the barrel. The angle formed along the line of my knuckles to forefinger is similar to the Parallel/Sector 1 angle and between thumb & forefinger near enough to a Teardrop/Sector 2 angle. I find it makes it a bit easier to visualize if I can approximate the angles somewhat.