Wrap a piece of paper around the tube. Lay it out flat and divide it into quarters drawing lines on it. Tape it back onto the tube and line it up so it's NSEW. Your eye should be able to do this easily by looking at it from various angles. Get a Sachs braze-on tab and drill a hole in the right line at the right height. The braze-on should drop right on and look great. Braze it up. I hope that makes sense. I'm sure there are fancier ways to do it, but this is how I would do it on a refit.
Craig