Simon when roughing out squares to circles on the bandsaw, drill a tight hole in a piece of scrap for a nail, and press it through. Turn it over so it stands vertical, point up. Clamp the scrap to the bandsaw table with the nail sticking up. set the distance of the nail to the blade to equal the rough radius you want. Then put a hole through the center of your blanks for a slip fit on the nail.
To use, put a blank on the stationary nail axle, and rotate the blank into the blade to cut the circle. This works best if you have a groove for a miter gauge in your bandsaw table, and put a tongue on the bottom of the scrap base so you can slide it in toward the blade, rather than just clamp it. You can then cut circles of any diameter, just by sliding in or out.
You can also cut cone segments this way by tilting the table to give an angle cut.
And you can cut rings this way, too.