Although the question I'm really trying to
ask is how do I know that a protractor is right for my turntable other
than the advice of anyone telling me use this protractor or that
protractor. How does any given protractor achieve the correct over hang
when not all turntable over hangs or tonearm lengths are the same.
Protractors are used with tonearms and cartridges, typically they are not turntable specific although a turntable manufacturer may include a protractor with 'table.
The variables to know
about an 'arm
are its pivot-to-spindle distance (P2S) and its Effective Length. Your
tonearm's manual or instructions should give you that information.
Correct P2S is critical. If it is not correct, no other 'arm measurement will be correct. Once it is set correctly you can forget about it until you change tonearms. Some protractors give the ability to measure P2S which will let you either confirm your 'arm's setting is correct or help you position the 'arm correctly if you are installing it. Some protractors include the ability to measure P2S, some do not.
There are protractors for specific tonearms. These achieve the correct overhang because they are made for a single 'arm. Examples are the WallyTractor and the Mint protractor.
And there are protractors that work with multiple tonearms. These achieve the correct overhang by providing settings for many different effective length tonearms. Examples include the Acoustical Systems SmartTractor and the Feickert protractor.
Cartridge alignment is not just about setting overhang, which is getting the stylus tip at an exact position. Alignment is also about positioning the stylus tangential to the groove and centering the cartridge's motor for optimal output from both left and right channels. Common practice is to align the cantilever against a set of lines on the protractor - getting the cantilever exactly parallel or matched to a specific line.. Some cartridge cantilevers are easier to align than others depending on how visible they are. A protractor with a mirrored surface with parallax compensation can help in such cases. The bottom-line here is some protractors are easier to use than others by helping your eyes.
Some protractors require you to align the cartridge at multiple points and/or trace an arc. Some ask you to align on a single point.
In theory you can get an optimal alignment with any protractor IF you know how to use it and are skilled with it AND your cartridge is well made. Alignment can be a finicky operation that can try your patience. Imo, a protractor that helps you do the job is easier to use and a protractor that is easier to use is more likely to get you a good result quicker. YMMV
There's actually a lot more to using protractors and alignment but this is too long already.
I've used the dbSystems, WallyTractor, the original Feickert and the Acoustical Systems UNI-pro which is the professional version of the SMARTractor.