That protractor is one seriously impressive looking piece of kit for sure.
Now stop and think a moment about what it is doing.
The hole on the left centers and fixes it on the spindle. The arm with the vertical rod precisely aligns the grid with the arms pivot point. This assures both precise overhang (axial mark) and alignment (tangential marks) with the extra parallel axial marks also aiding in setting alignment.
Okay. Great. Do this and you are perfect. No doubt about it.
But, wait! Why are there only these few alignment grids? Well, because the arm pivots. It swings on an axis. One degree left or right, everything changes. All your expensively and painstakingly acquired alignment goes right out the window!
This strikes me as an awful lot of bother for something you will only hear for those few seconds per side when the stylus crosses those null points.
Still, it would be totally worth it - if it made any difference we could hear. If we put on a record and it sounds funny, then better and better until right at the null point it sounds great! But then it gets bad again.... You get the idea. This never happens. Nobody ever in the history of playing records- we're talking millions of people, over a hundred years- not a one ever complained about hearing this.
Well, the tangential arm guys sometimes talk about it. They are so proud of their having a whole different set of problems! But even they never really complained about hearing the sound vary across a record. No one does.
I've tried. The velocity at the outside of a record is greater than at the inside. Higher velocity results in greater clarity, better detail. Always. This I can hear. Miniscule changes in cartridge bolt torque? This I can hear. No problem. Side bias? Yes, when it is dialed in the image is more solid. This I can hear.
Tracking error changing the sound across the record? This I cannot hear.
Can you? Would love to hear about it. While you're at it, would really, really love to know why you kept this a secret for so long?