There are different protractors that differ in the parameters they optimize, mainly how far to the LPs inside the tracking error is optimized. All geometrically "correct" protractors have in common that - independent of tonearm length - they cross the LP radius at two points with zero tracking angle. This leads to mainly two classes with either a ca. 63 or a ca. 66mm "zero point", the outer point is ca. 119mm vs. 120mm.
Protractors with only one zero point leave one in a bit of dubious state, one depends on a correct angle of the headshell (and ones manual abilities). So use at least a two point protractor and careful iterative setting and resetting of angle and overhang.
A 0.5mm or 0.7mm graphite refill tacked to the cartridge helps.
Actually such protractors have a third constant, which leads to protractors that are 1) aligned between platter and (horizontal) tonearm pivot points, and (platter fixed) zero crossing points that a have a fixed location on the platter! Such protractors make adjustment (in principle) much easier, it's then a 2) set the overhang - at a third point - 3) turn the protractor to one of the two "normal" points (fix it), and correct the cartridge angle to zero. If you want, I can send you a pdf of such a protractor.
More recently there is some sort of development of "evidence based" protractors market :-) because overhang in itself increases skating forces, and some try to reduce the overhang, leading to bigger errors with less overhang.
My experience is that with my three point protractor no tracing problem exist except by faulty cartridges, and the not so rare case of problematic second hand vinyl, caused by previous owners cartridges tracking at *too low* downforce. So check distortion with a pristine LP of known origin.
(Skating & anti-skating are not constant and a source of sonic problems)
Protractors with only one zero point leave one in a bit of dubious state, one depends on a correct angle of the headshell (and ones manual abilities). So use at least a two point protractor and careful iterative setting and resetting of angle and overhang.
A 0.5mm or 0.7mm graphite refill tacked to the cartridge helps.
Actually such protractors have a third constant, which leads to protractors that are 1) aligned between platter and (horizontal) tonearm pivot points, and (platter fixed) zero crossing points that a have a fixed location on the platter! Such protractors make adjustment (in principle) much easier, it's then a 2) set the overhang - at a third point - 3) turn the protractor to one of the two "normal" points (fix it), and correct the cartridge angle to zero. If you want, I can send you a pdf of such a protractor.
More recently there is some sort of development of "evidence based" protractors market :-) because overhang in itself increases skating forces, and some try to reduce the overhang, leading to bigger errors with less overhang.
My experience is that with my three point protractor no tracing problem exist except by faulty cartridges, and the not so rare case of problematic second hand vinyl, caused by previous owners cartridges tracking at *too low* downforce. So check distortion with a pristine LP of known origin.
(Skating & anti-skating are not constant and a source of sonic problems)