Michael,
There is no hard and fast rule to determine the amount of lead to use while shooting ducks. (besides a complex mathematical formula) The issue lies in the fact that there are too many variables in each shooting situation. Let me explain further, if a duck is crossing 30 yds in front of you travelling 35mph, assuming the velocity of your shot is a constant and you are swinging through the bird, and you are pointing 2ft ahead of the bird when you squeeze the trigger and you hit it, everything will change when the next bird is flying slower, faster, or in slightly different direction. never mind if you change loads.
The first step to determine lead is and understanding that your buddies are hitting more birds because their brains have enough experience to know where to to shoot where the bird is going to be, not where it is. This is an almost uniquely human ability by the way. Next do these three things. 1)everyday practice mounting your shotgun in the same way pointing at imaginary birds stepping towards your target with your left foot and swinging through 2) always swing through your target 3) practice practice practice In this way your brain will learn what to do.
Remember, quarterbacks use the same skill when throwing to receivers, we all have ability, some are just better than others