For smaller body tubes (up to BT-80) I use the Estes marking tool when I can(very handy little gizmo that thing)
Otherwise I do the old pi (the circumference of a circle thing) and divide by number of fins and then made several body wrap guides using thin styrene sheet and marked using a fine tip Sharpie (TM reg.)
I have a bunch of these in the old rocket "shop".These templates are also handy for marking lines around the circumference when setting up for painting and decal placement etc.
And I should add, instead of the door jamb routine ,I use aluminum angles of various lengths and right angle sizes (eg; 1/2 x 1/2 , 3/4 x 3/4 , 1 x1 etc.)Available at big box HW stores.I get mine from work.
Paul