tonearm mounting and alignment

I should have titled this "cartridge mounting and alignment." My mistake.

Doug, I appreciate the clarification. After searching through the forums, I realize I've been mounting cartridges poorly for years. I feel pretty stupid about that. I need a protractor. Do you have any recommendations?

Any opinions on Van den Hul's Cartridge Alignment Gauge?

I finally picked up a Turntable Basics protractor. What a slick little device! I appreciate the suggestion.

Doug, you're the bomb! Thank you very much for those hints. I was wondering how to accurately aim the line on the mirror at the tonearm pivot. I did my best by eyeballing it, but I'll do it again like you describe. A string makes perfect sense.

Regarding point #2, my cartridge is completely forward in the RB300 headshell slots and angled slightly inward just like you describe.

Now that I'm dialing in the cartridge accurately, I can't help but wonder whether my tonearm is mounted correctly. I know it's not because I mounted it and didn't measure the distance from the spindle.

Assuming your arm is mounted at the Rega-standard 222mm from the spindle

What points on the tonearm should I by using for this measurement?

Many many thanks!