For the handloader, I'm more comfortable with a 10% powder increase than I am with passing proof by visual inspection. Precise headspace gauges to detect any significant yielding are, I feel, more important than getting the pressure spot on.

I would, though, insist on chronographing the loads to be sure the fps indicates a fair rise in pressure. Looking over published load data, it appears fps rise gives you a much better indication of pressure rise than does charge increase.

Without question, both approaches are flawed, and I can cite examples of both not working as expected. Still, I think either is better than no proof load at all.

Beyond that, the handloader must resort to using strain gauges. The T/C Encore makes an excellent platform for such work.

If you can find a lab with a piezo barrel, send them cartridges each with a bit more powder than the previous in the series and have them tested in sequence until a certain pressure is reached.