Some sort of step function? (Nothing like making it complicated...) With a 4+ inch barrel, within 5% of the average velocity, with a 3 inch barrel, within 10%? [sigh] Pretty soon you'll have a formula that requires a computer.
One of the things that I always liked about the Pro-Am is that they don't have a power floor at all---they don't care what you are shooting. EXCEPT that they say the steel must fall to score, and if it didn't fall, your ammo is too weak and it is just too bad. No reshoots. (Of course, they can do that because all of the targets are steel.)