Boys, boys - we have protractors right here!https://www.vinylengine.com/cartridge-al ... tors.shtml
Check out the Tools link at the top of the page.
Arc protractors must be matched to the mounting distance, spindle to pivot, of the arm. You can also generate a custom one using Conrad Hoffman's neat program.
And the 2-point protractors can be used with any arm.
With a 2-point protractor, align the cartridge perfectly on the outer grid, with the stylus on the correct point.
Move it to the inner grid, moving the protractor, and place the stylus on the correct point.
There are 3 possibilities:
1 - It is perfectly aligned. You are done.
2 - The cartridge angles outward: move it back a bit, and try again.
3 - the cartridge angles inward: move it forward, and try again.
Keep them spinning.