Probably teaching egg sucking, but just in case:
1. Get a piece of A4 paper, fold down centre of long edge.
2. Mark where cut is going to be.
3. Wrap folded edge of paper (which will be straight) around pipe, pinch end with fingers and align with mark.
4. Draw round pipe where edge of paper touches. Should give you a straight line round the circumference.
5. Cut 1/2 way through, rotating pipe as cut deepens.
6. Go back and cut again, all the way through.
7. Use a Stanley knife blade to chamfer inside and out and bevel outside.
In the past, I have used a split piece of polypipe with a known square cut end to do similar, but I can see using the paper would let me mark up pipe "insitu" so thanks for that....
The back story to all this is that I had a plumber out to sort an issue with a leaking down stairs loo (which he did) and while in the bathroom, I mentioned the waste from the basin would occasionally leak...
He had a tinker during which he loosened the waste pipe off before fixing it again.
The pipe goes from the trap below the basin out through the exterior wall and then drops vertically a foot or so into another waste pipe.
Somehow while twisting tightening the pipe inside the house, a joint out side has failed, and pipe is no longer in the correct alignment to fit back in...
To compound this, outside the house there seems to be two slightly different size of waste pipe involved, one being glued and the other a push fit and it looks like the joint that failed was the one where they were botched together, probably during the original install...
Given the internal pipe is glued, I need to basically strip it out and start again otherwise I am going to end up with a jigsaw effect of different joints and bits of pipe..