That?
I actually don't get it. Unless you are able to align the microscope precisely perpendicular to the stylus, then parallax error will cause your angular measurement to be incorrect. And how could you ever be sure it is aligned precisely? It seems impossible to me. Perhaps it's a good way to get in the ballpark and then finalize by ear, but it doesn't seem like a better way than any other method for the same.
I agree and had the exact same thought: how do I ensure the microscope lens is perpendicular to the stylus. I suppose you could say it as how do we get the lense parallel to stylus.
One approach could be to have an object behind the stylus. If on either side of the stylus that object was in equally clear focus then the lens is roughly? perpindicular. Consider this photo I took of a Lyra Etna:
Behind the stylus is a a piece of lead from a mechanical pencil. I thought that was pretty good.
But... then I noticed the horizon above the lead appeared very slightly sloped as did the lead itself, suggesting the lens might be a hair off of perpendicular - maybe - it might be the lens is perpendicular but slightly angled - the round lenses on thes USBs can be tricky to get level. That neither validates nor invalidate the notion of using something behind the stylus to gauge lens perpendicularity. I think part of the problem (for me anyway) was being able to make really fine adjustment to the lens position. None of the stands or mounts for these small microscopes seem up to the job, at least that I can find, allow micrometer type adjustment.
The other problem I find is with using the microscope's software to discern the angles of the stylus. Being off by just a small amount (a pixel?) will change the measurement of an angle. Again an object behind the stylus may help some. The line between the pencil lead and the CD it sat on should give a common base for one leg of both left and right angle. Of course that base should be parallel, which in the picture it may not be. You still need to draw the line that creates each angle and the point of each angle must proceed from the same spot.
Once you get a measurement and if its not what you want, you need to adjust the tonearm height - another possibly v. fine adjustment - then measure again. Of course if you subsequently change VTF, you may need to start over.
Another issue is where exactly on the stylus should the angles be measured. The Etna stylus is a piece of cake but with exotic stylus shapes and cuts, its not always clear.
In my experience it takes a lot of time, patience and a moderately steady hand. But, for now, I think it is at least theoretically possible to use an USB to set the stylus at a specified angle.
I have no idea if this is a correct result, but there it is.
This yields an SRA of 91.8°. Benz LP S cartridge.