What you want to do is divide the end of the pipe into thirds. So if you're cutting down from the mark you aim for one third down on the end of the pipe. If you're cutting up from the end then you start a third down and cut up to the mark.
I don’t think I understand when you say you divide the pipe into thirds. Do you have a picture of how you lay the pipe out?
This is my understanding of laying a pipe out into thirds.
These are "my" views based only on “my” experiences in “my” little bitty world.