Well the lawyer in me would say that if the bird was flying at "40 mph" then the answer would be he is flying at 40 mph, but I understand your point. But one thing to consider is a duck decoying, is that they almost always will come into the spread against the wind when possible, not with it at their tails. Pass shooting would be another issue, with the tail wind giving them more speed one would expect. Teal seem to be always traveling at what I refereed earlier as mach 3. Landing with a strong tail wind is more difficult for a bird, while landing against the wind is much easier.
But even at 60 mph, the assumed maximum of your example, the lead goes from 5.866" to 8.799". Still not that big of a deal. I will continue to consider shot string to be of little or no help in killing birds. Pattern density and proper centering of the pattern on the bird will be much more effective, than hoping a shot string will makeup for my margin of error. Might be 99% of the chance for a clean kill, string 1%. Or in plain English. When I miss I do not think the shot string will bail me out. I miss.
