As I don't own one, I don't have a vested interest in promoting SME or otherwise, but I have been sent a pic of their protractor for the SME V, so I can comment on it.
So let me say that SME owners can be assured that the concept is not at all crazy, nor inaccurate.
Although, like all protractors, it depends on a number of things.
The first is that the SME arm is correctly designed and the headshell is as specified at the appropriate angle for a Baerwald IEC alignment at a particular nominal effective length (as given in the SME spec for their various arms)
Hopefully this is so. Then, as the quality of SME manufacture will be top class, we assume the arm is made to the specification.
The second thing is that the cartridge and therefore the cantilever is set exactly on the headshell axis. For this a depth gauge should be used to align the the cartridge body, if it is cuboid, although the cantilever is a problem, as always (but that should really be an issue for cartridge manufacturers). So it is possible to align the cartridge to within 0.05mm along its length, ie 0.15 degrees or better. This is at least quantifiable, not guesswork.
The third thing is that one has to align the arm tube edge with the outline of the arm tube on the card. This isn't so clear - they should have omitted the outline of the arm entirely, and simply drawn some closely spaced lines at the same angle as the arm outside edges, as the principle is to align the arm outside edge parallel with a line, any line, at that angle when the stylus is on the null point. If a cartridge has a longer (or shorter) cantilever, the angle of the arm, and hence the edges (even though they taper) remains the same but moves sideways.
Using the arm tube as a guide, it is possible to obtain an accuracy far greater than with a cantilever view - perhaps as much as a factor of 10 (assuming generator on the same axis as cartridge and cantilever on same axis as generator.... we have to assume something ...!)
The fourth thing is that the protractor is made accurately.
So, therefore, the potential for correctly setting up an SME using their protractor is very high. The areas where it is lacking are the same as other protractors, namely the accuracy of manufacture of arm and protractor, but it scores in angle alignment accuracy, as in this area it is potentially more accurate. Also any errors in the protractor will be minimised by the fact that the arm is optimising alignment at the base not at the headshell.
Of course it never hurts to double check using an accurate two point protractor with suitable nulls. But this applies to all protractors.
