I do this quite a bit and thought I'd add what I could to help. It it's a striaght pipe like in the pick, you can use two pieces of angle iron on both sides of the pipe. It will make sure the pipe is perfectly striaght. Unfortunely, you also get to see how crooked you cut it.
Also with the curved pieces I like magic markers. I throw the piece up there and draw 2-3 lines acroos the cuts. Take it all off and take it to the bench and line 'em back up. Hope it helps.