Tonearm Overhang and Tracking Error

Since the old tonearm I bought at an auction naturally didn't come with any manuals, I searched online to gather information.

SME tonearms don't seem to specify an overhang, but instead use a sheet for adjustment called an alignment protractor.

I created one in CAD while checking the dimensions of each part.
Doing it yourself makes it easier to understand what values like overhang and offset actually mean. 😺

The positions of the two null points and the curve of the error angle change depending on the distance from the spindle to the arm pivot and the length of the overhang.
There seem to be several schools of thought, such as aiming for an average or prioritizing the inner grooves where conditions are less favorable, but SME's inner null point of 60mm seems to prioritize the inner grooves.

If you take two null points on the arc traced by the stylus around the arm's central pivot and draw two circles with the null points as radii centered on the spindle, the tangents at the null points would not normally be parallel.

Therefore, for protractors where the inner and outer null point gauges are printed parallel on a long, thin sheet of paper, you have to rotate the platter slightly to align the position.

I feel like this simplified protractor, which makes them parallel, makes adjusting the overhang harder to understand.

I've placed the PDF here, so if you use the same model and are interested, please feel free to use it. 😽

For those who find making one in CAD too troublesome, there are also websites that will automatically generate one if you enter the numerical values.