Here is another trick. Go to a couple of garage sales and buy old white sheets. (Using the wifes good sheets will get you in trouble. ) Now, hang them full out at say 25 yards and spray paint a dot on the sheet. On a twin sheet, you can put two and on a queen or king, four. Now, load up five shells and take all five shots at one dot with the full choke and lead shot. Do this like Griz said, don' t aim but pull up and shoot repeatedly. This will put a good size tear in the spot you are hitting. Repeat for each choke. Then go back to 35 yards and do it all over. Same at 45. This will give you a good idea of what kind of point of impact you have. Which is what you really want to know.

Vapodog is about 90% right in the proper way to pattern your gun. Proper is to take the shot at a center mark and then circle the center of the pattern with a 30" circle. This will give you point of aim and point of impact, (If you look at shooting or hunting mags that show pattern circles, their is always a dot indicating point of aim.) but I don' t know that you necessarily care that you got 65% of 72% of the pellets in a 30 inch circle.

The top method will tell you alot w/o alot of counting of pellet holes.

I do beleive it is 70%, 60%, and 50% at 40yards. Skeet is measured at 25 yards.