It is all about geometry, the two null points have zero tracing error and if the set up is off the null points will be different than what we think we have. If bad enough the inner null point could be on the record label for example. A tracing arc with a specific distortion curve should have the same geometric error and distortions regardless of the arm, cartridge stylus or if the record is stereo or mono, I thought?
You can't compare tracing error distortions to amplifier THD specifications, we all know there is more to the sound of an amp than THD figures, but picking a tonarm/cartridge set up geometry that has more tracing error will produce more tracing distortion, unless the mathematics used to figure these distortions are a lie. Or maybe a little extra distortion sounds better to some people.
Anyway, a protractor like the DT, if it makes precise alignment easier is a good thing for those who want to buy it. Getting close to perfect alignment even with the best tools takes time and patience and with practice gets easier and sometimes a little luck makes the set up go faster. Moving that cartidge in such tiny amounts and the eye strain always tests my patience. I still use a MintLP and some arc protractors I printed using the vinyl engine template generator.
