I have a large cardboard cylinder 10 inches in diameter and about 5 feet long. The cardboard is about 1/4 inch thick and it weighs about 30 pounds. I want to cut a bit off one end so that the cut end is a perfect circle whose plane is *exactly perpendicular* to the axis of the cylinder. Since the cardboard is so thick, I think I will have to cut it with a saw.

What is the best way to mark the cut line as accurately as possible? I know about the trick of wrapping a large sheet of paper around the cylinder and marking along one edge. Is there a better / more accurate method?

ETA: The current cylinder end is jagged and irregular, so it would be pointless to use it as a guide.