Hopefully your point of impact is the same as your point of aim.
The correct procedure listed for patterning a shotgun is a 30" circle at 40 yards with the shotgun in a rest. This is for a true pattern. Trouble is you need to know how many pellets in the load and then count how many in the circle.
There is a downloadable chart that you take a picture of the circle with your phone camera or any digital camera and put it in this program and it tells how many pellets are in your downloaded data. I haven't tried it yet but from reports of others, works great.
http://www.shotgun-insight.com/There is also one to buy that looks very good to have.
http://www.targettelemetrics.com/
