I have an edge gauge like this. I just went and played with it a bit. I was surprised what a big difference one degree was. With a good back light, it is easy to tell which notches are too acute and which are too obtuse. So much so, that I think 1/2 degree would be easy to tell, and maybe even 1/4. The trick is to swap between front and back illumination so you can tell which part should be dark.
So I tried to lift the gauge a bit to reference from the base part of the edge, rather than the base itself. The relief notch in each corner uses up a good fraction of the base edge thickness, making it tough to sight on that. Might still be possible with a helper to switch the light. Take the difference between metal-to-edge and ptex-to-edge, and that is your angle.
But this experience makes me believe that something as simple as this can measure base edge to a sub-degree level, if the design was tweaked a bit.
I think the first thing I would do if I was serious about making a bevel measurement would be to put something behind the gauge, resting on the base, to block light between the gauge and the ptex when measuring the base metal. And maybe rig up a soldering "extra hands" stand to take the wobbly hands out of the loop.
So I tried to lift the gauge a bit to reference from the base part of the edge, rather than the base itself. The relief notch in each corner uses up a good fraction of the base edge thickness, making it tough to sight on that. Might still be possible with a helper to switch the light. Take the difference between metal-to-edge and ptex-to-edge, and that is your angle.
But this experience makes me believe that something as simple as this can measure base edge to a sub-degree level, if the design was tweaked a bit.
I think the first thing I would do if I was serious about making a bevel measurement would be to put something behind the gauge, resting on the base, to block light between the gauge and the ptex when measuring the base metal. And maybe rig up a soldering "extra hands" stand to take the wobbly hands out of the loop.