What's a geo-disc?
I have printed out a whack of different protracters, and I'm not able to tell by eye if it's better aligned at one point or another within a few millimeters of radius.
While I can appreciate the science of the difference between the various alignments, I still just use a hole-punched business card, with one line from the centre of the hole out, and some 90 degree lines drawn across. Sometimes I'll draw another line parallel to the centre line that aligns with the end of the headshell, so I don't have to keep bending over to check the tip.
If anyone can get it more precise by eye, given the tolerance of tip construction, then kudos. If it looks straight at every point across the record then I'm happy.
Ear only after on the inner grooves and don't ever look again or I'm sure to "fix it" wrong.
I suppose with a 'scope and some mono sign waves I could see the errors, but once you have the overhang correct for the arm length, it just a matter of twisting to set it right. I suppose that on the outer groove there is a difference, but it's very vague. The tiny inner grooves are where it counts for things like VTA, azimuth, and alignment IMO.
And the best sounds are usually in the last grooves of a side grrr.