"Don Foreman" wrote: Clamp tube to bench. Set height gage for 1/2 tube
dia. Scribe both
sides of tube.
^^^^^^^^^^^^^^^^^^^^^^
Or: Measure tube circumference accurately. Set mark half-way around. To
check accuracy, do it again the other way 'round. If the two marks do not
agree, go half way between them.

You could check accuracy on Don's method by turning the tube over, just in
case your height gauge setting isn't spot on.