Circumference Marks on a Tube

It's often necessary to draw a guide mark on a body tube to position a centering ring or other construction element. The easiest way to do this is to make a single mark at the right position and the wrap a piece of paper around the tube at the mark. Make sure the edges of the overlap line up and the paper is tight around the tube and you'll have a line is perpendicular to the length of the tube!
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