My answer will parallel Wolfkiller's. I most likely will make the decision as I am looking through the scope, as my mind has already went into "kill mode" and it is now "show time". A large factor will be whether I am shooting from a rest (a nice aspen tree) or off-hand. If my mind can play a "tape" from past experience that makes this feel comfortable and it feels like a high probability, then the gun will go off by itself and I will hear no muzzle blast and feel no recoil. The lead, if any, will be done subconciously.

100 yards, 20mph (pretty fast and under duress) --- I'm guessing the gun will NOT go off. If not, then I will try a cow call though, it may open up a 3 to 4 second window of "stop and look" and then "automatic pilot" is quite important at that point.

Good Luck and Good Hunting,
EKM