Shotgunners that are concerned with choke performance know to test their shotgun/choke/shotshell combo using the pattern board. There just is not a better way to do it!
The usual way (as all the books/articles, etc. state the procedure) is to set up the pattern paper, step off 40 yards and take the shot. Using a thirty inch circle to encircle the densest portion, you count the pellets and figure the percentages. Using a Cylinder choke (not really a choke, but actually an extension of the barrel bore), the percentage count is supposed to be in the order of 40% at that range. For Improved Cylinder, the percentage is 50%, Modified at 60%, and Full at 70%. Just remember, these are averages and your milage may vary!
Frankly, I don't know any shooter that blasts away using an IC or MOD at forty yards and expects good results! So, let's look at doing pattern testing using a slightly different method.
Why not simply test CYL chokes at 25 yards, IC chokes at 30 yards, MOD chokes at 35 yards and FULL chokes at 40 yards. At these different ranges, the pellet percentage should be near the 70% pellet count for each choke! By using the right choke for the distance, your probability of hits goes up which results in more clays destroyed and more game in the bag. The shooter will have to pay more attention to the distances to the target/game, but the results should be worth the effort.
And, by investing in BlackBore chokes (made from 17-4 PH stainless steel), the shooter knows the choke will work for him/her every time, stopping the wad from "rear-ending" the shot column and eliminating fliers!