I have no idea just how much precision is required to get the most out of an arm cartridge combination. Obviously, it doesn't hurt to optimize the geometry, but, it is not clear to me how much is lost by being slightly off. This is particularly the case when one finds out how much imprecision that is built into analogue playback in so many other respects. You can align the cantilever precisely with the lines on the protractor, but, when in play, forces, such as skating forces, pull the arm out of alignment. Perfect alignment only optimizes geometry if the zenith of the stylus is perfect (i.e., if the stylus is mounted on the cantilever such that the wedge of the stylus is perfectly 90 degrees to the cantilever edge). It turns out that it is very hard to get the zenith correct and many very expensive cartridges have styli that are misaligned. The only way to sort of compensate for this is to have the stylus examined under a very specialize microscope and then deliberately mount the cartridge with the cantilever out of alignment enough to compensate for the zenith error. This is extremely complicated, and can only be analyzed by specialty companies, like Wally Tools.
There are even tonearms on the market with NO offset angle at the headshell, so they end up having massive errors when it comes to aligning the cantilever as close to tangent with the groove. The theory behind such arms is that this geometric error is of less importance than eliminating the skating forces caused by an offset angle.
The upshot is that being ultra-precise about alignment may not have that much practical benefit, and it might well be the case that a cheap protractor with thick, imprecise lines and no way to establish the perfect viewing angle is still good enough.