One of the more useful uses for the mirror protractor I have found is for azimuth. With a mirror you can really see if the cart is level, it does make this more obvious.
Yes, you can see if the reflection of the cantilever is in the same vertical line as the cantilever ... but that only gives you an approximation
that the stylus
is riding level in the groove (as it presumes the manufacturing process of glueing the stylus to the cantilever is 100% true).
From that foundation, you then need to:
* adjust by ear, so that the height of the sound stage of a symphonic recording is the same in both speakers
* you use a test LP, a narrow band-pass filter centred around the 1Khz tone which the test LPs use, and look at the crosstalk reading on a 'scope. There is some complex ratio which takes into account possible differences in output in each channel of the cartridge, which enables you to optimise the azimuth.
PS: I just use the mirror and then adjust by ear!