I had some crazy fittings to cut when I caged my Mustang like angles going into curved tubes in the halo. I found plastic pipe with an inside diameter the same as the outside diameter of the tubing I used and cut it to fit by hand. Then I rolled up paper and slipped it into the plastic and traced the cut and wrapped it around the steel to transfer the pattern. I used a hole saw based tubing coper on most of the fittings and it worked out ok. I broke down to basic math and geometry and measured angles with a protractor and plastic tubing for mockups.
I also found a tubing coping calculator on-line that is in the link below. The idea is to find the angle of the intersection of the two sections of tubing and enter it along with their diameters (usually 1.625). It gives a curved line that you can download and print at 1:1 scale and cut out. Then wrap it around your tube and trace the pattern. It's weird but it works.
Link:
http://www.metalgeek.com/static/cope.pcgi
Dale