tonearm mounting and alignment

That’s great. Don’t forget warning #2 in my post above. Assuming your arm is mounted at the Rega-standard 222mm from the spindle, proper use of the TTB will leave most cartridges well forward in the headshell slots and angled slightly inward. That’s okay.

BTW, the toughest part of using the TTB is accurately aiming the sight line at the tonearm pivot. Unless you aim it “perfectly” your results will be meaningless, but the aiming is tricky.

The best method involves taping a piece of thread to one end of the TTB’s sight line, at the side away from the tonearm. Now drop the TTB onto the spindle, aim it roughly and then pull the thread lightly taut directly over the tonearm pivot. Sight vertically down through the thread to the line, using one eye only. Swing the protractor L and R a few degrees and you’ll see the thread, the line and the reflections of each move in and out of alignment. When ALL FOUR images are lined up, your protractor is aimed just right. Now position your stylus on the outer alignment point and square the cantilever to the grid.

It's much easier to do than to write, so don't worry!

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!

The dimension is called "spindle to pivot", and it means precisely that: from the center of the TT spindle to the exact point around which the tonearm pivots (horizontally).

The tonearm's horizontal pivot axis is easier to see on some arms than others, and it's not obvious on a Rega. On my old Rega style arm I found it useful while aligning to stick a small square of scotch tape on top of the bearing cage. I then marked a dot on the tape with a pen and eyeballed the dot from above whilst swinging the arm back and forth. If the dot moved it wasn't above the pivot point, so I nudged the tape and tried again. Cheap trick (like the thread) but simple and effective.

P.S. Make sure your stylus guard's in place during all this stringing, taping and swinging!

P.P.S. If your table allows, try nudging the arm a skosh closer to the TT spindle. A pivot to spindle dimension of 220-221mm will let you align using the TTB without shoving the cartridge all the way to the end of the slots.