Do you have a chop saw or know someone who does?
Cut in on a chop saw. That's what we do in the construction business if there is one on the job site. A regular wood blade cuts it fine, with little or no extra wear to the blade.
Sheldon
Wrap an 8 1/2 x 11 sheet of paper tightly around the pipe and tape it in place . This will give you a guide to follow when making the cut. If the cut does not turn out to be perfect the paper can serve as a guide to file or sand it straight.
Regarding the 2/22 Build a cement silo article, how did the author cut the PVC pipe perfectly square? I've tried this in the past and failed after numerous tries.