Azimuth 2020

Peter Lederman is the last person I would take advice from. As lewm suggests it is a record wear issue not a crosstalk issue. Crosstalk goes to how well the cartridge was constructed. The the crosstalk is not at it's highest value with the stylus perfectly upright in the groove then toss it. It's junk.
Next. Springstein, I am not at all sure what you are talking about. The only really important factor in regards to the mirror used is it's thickness which should be about the same as a 150 gm record. Why? The vast majority of us have offset tonearms. Because of the offset as the arm goes up the stylus leans toward the spindle. Azimuth in offset tonearms is elevation sensitive. Those with tangential trackers can use a coke bottle if they want. As far as what side of the mirror is plated? It does not matter. As long as the stylus is at record level and the reflection is perfectly symmetrical you are in business. 
Some people try to just eyeball the stylus. Bad idea. The stylus has glue around it which is never even and it can fool you. The mirror greatly magnifies the error and allows you to focus on the very tip of the stylus.
You need good lighting! Try it and you'll see. Just grab one of your wife's compact cases. There is a mirror in the top. Also don't forget to defeat the anti skate or the arm will slide backwards!

Lewm, that is correct it is just measuring the amount of leakage across channels and it does allow you to optimize this. As you have noted the problem is this does not guarantee a perpendicular stylus and the thing cost what, $300? A pocket mirror guarantees a perpendicular stylus and costs maybe a buck fifty?

Peter Lederman is the last person I would take advice from

whoa!

@mijostyn , "The the crosstalk is not at it's highest value with the stylus perfectly upright in the groove then toss it. It's junk."

No disrespect Mijo, but you wouldn't be thinking of dumpster diving, would you?

I know there are several devices for azimuth.
I am not very tech able but found that the SmartTractor was easy, accurate and made it obvious.
After a couple of steps it uses a small magnifying glass that when looking through you can clearly see how to adjust the cartridge so that the stylus tracks at the right angle to the grove. About a millimeter to the left and the difference in the sound was huge!I was cautious at first thinking it was too complex. But it as a piece of cake. I borrowed the SmartTractor from a friend. Too bad it cost about $700. But for such a useful, multi purpose tool it is worth it.