What Niel and Sam said. I know in the video series heavydutysg points out that most of the razors sent to him by members who couldn't bring them to shave readiness needed further work to adequately set the bevel.

The magnification and the TNT help me determine that. The TPT follows as I progress up in grits. When you say shave arm hair, I assume you mean with the edge suspended above the skin ? If it is touching the skin it isn't a reliable test IMO.