For pattern percentage results or choke proofing, count all the pellet holes inside your 30 inch circle. Now you need to know how many shot pellets were in the shotshell you used, try here first.
Simply cut open one of the shells you have left and count the pellets.
Divide the pellet hits in your 30 inch circle by the total pellets in the shotshell that you were using and this gives you your pattern percentage for the choke you used. Do this with all your targets add the aggregate for overall average. Choke Determination:
Idealisticly we are looking for shot and choke combinations that produce the following results when patterning.
Extra full choke delivers a 30 inch, 70% pattern at 45 yards.
Full choke delivers a 30 inch, 70% pattern at 40 yards.
Modified choke delivers a 30 inch, 70% pattern at 35 yards.
Improved cylinder choke delivers a 30 inch, 70% pattern at 30 yards.
Cylinder bore delivers a 30 inch, 70% pattern at 25 yards.
For a more detailed shotgun pattern analysis, try here.