Here's a quick video of a super easy way to cut perfect circles on the bandsaw. It's just a wooden surface that goes over the bandsaw table and is cut through halfway. I glued and screwed 2 pieces of wood underneath as stops so that it lines up in the same place each time.
Then I drew a line across from where the bandsaw stops on the piece of wood, and that is where the pivot point will be placed.
To cut your circle, find the radius you want and drill a screw threw the work piece. It helps that one length of the work piece is already to size. Press the screw into the wooden surface along the line that is perpendicular to the blade. As you cut the work piece, rotate it using the screw as the pivot point and you will get a perfect circle!