Hello gawelder, it really depends on the system you are employing to do the notching. If you are using a paper pattern or some other sort of pattern that has a center line on it. I would probably apply a centerline on the pipe from end to end (using a piece of angle iron that saddles the pipe and then drawing a line along it's edge works pretty good), then center your pattern on this line before you draw the notches. If your pattern is only designed to be used to draw one side at a time I would use the angle iron to draw two lines 180 degrees apart from one another. Draw your first line, then use a flat tape and measure from both sides of this line to find the point that is directly opposite of it on the other side, then draw that line with the angle iron. Now use the pattern and the centerlines to draw the notches on both sides and at both ends of the pipe. If you have some sort of mechanical notching system that can notch the ends I would first notch one end and if you don't feel comfortable about eye-balling the other end, I would use a short section of pipe to fit into the notch that you have done on the one end and when you turn it around to do the other end put a pocket level across the square open end of the short section of pipe that you fitted into the notch and rotate it until it reads level and then do the notching for that end. the short piece of pipe should be located vertically in the notch. Without knowing more I don't have any other suggestions at this time. Give us a little bit more information and keep us posted on your progress. Best regards, Allan
EDIT: you can disregard part of my post, I made it before I read some of these other responses.