I thinl labman has an intersting idea. Let's try to refine it further. Let's for th moment say you have a piece of pipe cut at the correct angle and you need more.
Take a piece of stiff paper and wrap it around the pipe. Secure with post-it tape. Cut it so it just fits. Trace the pattern of the cut on the paper. Take off the paper and cut it where you made your mark. This then can be used as a pattern to mark other pipes. Remember to use the seam as a reference point.
I'm not sure how to make the pattern in reverse. Knowing the angle and the diameter of the pipe. Maybe the math guys can help.
You can always make a pattern with a piece of PVC of the same diameter. Cutting that with a hacksaw would be easy and then use that to make the pattern to cut the metal pipe.
Layout fluid might me a better choice for marking. One source is here: DYKEM LAYOUT FLUID at Brownells
It's the standard in metalworking and is also available in a spray. It will wash off with methanol. You pint the area and let dry which is about 2 minutes. Then you use a scribe to create a line in the layout material.
You could also try, once you have the pattern, painting the portion to be removed. Let it dry and then remove the painted portion of the pipe.