"No human eye can accurately set a stylus perfectly at 90 degrees inside a record groove. You can get close visually, but there will always be microscopic deviation. And that tiny deviation matters far more than many people realize."
Doesn't matter, you can do it visually with a $15 tool and it can easily be verified with a fixed frequency LP test track with each channel recorded out of phase. Simply throw your preamps mono switch and if the azimuth is correct, you'll have a complete "null" or silence. A bayonet head shell usually has enough play once the collar is loosened to make the necessary adjustment. Other tonearm designs with fixed head shells usually have a set screw at the back of the arm near the gimbal so it can be rotated slightly to make the adjustment. You don't really need to over think these things or spend!

