Once in a while, we need to cut square ends on tubes, and a lathe isn’t available. The way I mark them is to simply wrap a piece of paper around the tube, tape it, and use that as a marking guide. It works well enough that I was able to cut some tubes square enough to use as aileron pushrods.
In this case, I also took the opportunity to lay out some properly spaced marks for drilling holes later. These are the tick marks on the paper. Remember, the paper needs to be longer than 3.14 x diameter so that it overlaps.
I have a lathe handy, and I just use a rotary miter saw with a carbide blade to cut aluminum tubes.