Yes, I'm going to echo what everyone else said.
First, get a large 30X30 piece of paper. (office max has rolls of brown paper)
At 15-20 yards, sight the red dot in with bird shot.
Back up to 40 yards and shoot your desired load. Fine tune the red dot if needed.
When checking pattern density, put your 10" circle on the most dense part of the pattern no matter where it is in relation to your bullseye