If you must make a more precise mark for a cut... use a pipefitters wrap-a-round... or do this (which will perform the same function as the pipefitters tool)...
Take a Manila folder, open it, and cut a +/- 4" strip of it along the longest edge. Make a small mark on your tube where you want to cut. Take the FACTORY (straight) edge of your Manila material, and place it at your mark. Wrap the rest of the Manila around the tube so that the manila's factory edge lines up with itself all the way around the tube at your mark... Make sure you wrap the entire length around it, wrapping the manila over itself, and keeping the factory manila edge lined up with prior wraps and your mark. Doing this will ensure the mark (and subsequent cut) is perfectly perpendicular to the length of the tube, and will yield the best result humanly possible.
I would then make the cut with a wafer wheel on a right angle grinder... carefully of course. I think that would be the most commonly available tool that if used carefully, has the least chance of distorting the tube during cutting.
If you have to use a saw of some sort, lubricate the teeth with a generous amount of wax (dubbing wax or toilet ring wax should work fine). This will prevent the saw teeth from clogging up with aluminum and sticking or binding up during the cut, which may bend or kink the tube during cutting.
De-burr the cut edge and reattach the cap as these other guys recommended.