When cutting a mast tube or any round item the tape method is good but i would try and use a card or stiff paper that is wider than tape. If it is about 4in wide or more and long enough to rap around the tube, one and a half times or more the better.
This way you can put the edge your going to use to mark the cut line, over itself and beyond, making sure the top marking edge is on top of itself all the way round. That then will make sure the line is not wavy around the tube !
Did I put that right??