Never had a chance to try this one but I've heard that empty toilet paper rolls make great templates for angles to cut in tube work.
Another thing to do, is to use extra cut off tube peices to make "blanks". Notch one end with a 10* and the other with a 20* and so on with more cut offs. Then you can use them like "feeler guages" to estimate notch angles.
yeah thats a good one. Also if you can get the tube in the ID of the OD your are using you can then trace the cuts on. Assuming it can slide over it easy enough.
other than that, a helper is always good with larger pieces you are welding/cutting. yeah its cool to do it all yourself but you'll feel like an ass when its fuckn crooked.
keep lots of clamps and arrow magnets handy for keeping things in order while you weld/cut.