With a modern gun or caplock and decent load, I use the swing-past/slap the trigger method of shooting flying. Thus as the front of the tubes touches the bird/slap. With my 20bore flintlock pelter, using 1 1/8oz 7 1/2's and 82gr(3drams) 2F, I slap the trigger 1foot past the bird, continuing to swing of course. The 20 shoots modified patterns from it's IMP/CYL choke with that load.
The amount of "leade" will depend on the speed of your locks and, your tuning ability. Some locks "like" just barely the bottom of the pan covered with priming powder while others like the pan filled. Experimentation is a must.
Some locks are "much" faster while others have slower ignition. This will shorten or increase you leade.