The amount of "required lead" is how far the gun must be in front of the target when the shot leaves the muzzle. That's the mathematical fact. What you see or perceive may be something quite different, however. What it looks like to you, when you center-punch the target is what really matters, as Mike said. That's the picture you want to keep in your memory. A really fast swinger may "see" no lead at all and a shooter with a poor follow through(like me) may "see" 6-8 feet but 3-4 feet has to be there or the bird will be missed(behind, most likely).
