I use empty toilet paper rolls and paper towel rolls for notching tubing since it holds up for a lot longer. If you can figure out the circumference of your tubing (diameter multiplied by 3.14), you can divide the stretch out by 2 and mark the cardboard roll in halves. At that point those marks can be used for getting more consistent notches that are exactly 180 degrees apart. Any present weld seams in the tubing (if equipped) can help when clocking your notches too.

I'll be honest here. As a sheet metal worker, I always had an excess of used Wiss snips laying around, and I used to use those for cutting up to 16 gauge capacity (mild steel or stainless) tubing and conduit. Now I use the hole saw and drill press method with a decent vise, since snips are too hard on my hands.