How critical is the exact placement of the tonearm

It is amazing how good a tonearm/card will sound even if pivot-to-spindle is off by only .03 mm or so. But it is amazing how much better it does sound when you reduce that error even more.

Contrast these responses with the instructions for mounting the 47 Labs (or Miyabi?) RS-A1 tonearm, which is merely placed on the armboard so that the stylus is "about" a cm from the spindle!

Lewm, the 47 Labs arm is quite different due to the rotating headshell. You cannot (or better you don't have to) adjust that arm for the standard Baerwald or Lofgren geometry anyway. The rotating headshell takes care of that.

For a standard arm (with fixed headshell), if your spindle to pivot distance is far off, you cannot adjust the overhang correctly anymore and fulfill the standard Baerwald or Lofgren geometry requirements (two correct nullpoints across the arc).

Good point. I am aware of the reasoning, but I just thought it was a point of interest. The RS-A1 is eccentric in many ways, but somewhere in there is a very good idea.

If you're close you can make up the difference with overhang adjustments most of the time.

Once I installed a RB300 Tonearm on a Linn LP 12. The armboard on the Linn was drilled for a Linn tonearm. (I only did this as a stopgap until my new Linn armboard came that was pre-drilled for the Rega tonearm by the way)

I think the two are different by 7 mm or so and the Linn arm hole is larger.

I moved the Rega tonearm around in the larger Linn armboard hole until I was about 3 to 4 mm off. Then set the overhang so that it hit both marks and to my surprise it played just fine.

No inner groove distortion, no odd sounding bass or high or mids or anything out of the ordinary.

About 2 days later my new armboard arrived and I quickly replaced armboards and set up my table again. I can't say that I heard any difference at all.

Hitting both marks on a two point protractor makes one very huge assumption. That is, the pivot to spindle is dead nuts on. If it is not, there is nothing you can do to correct for it as the tonearm swings the arc. Sure, you might hit it at one point and it might be close a little farther into the swing but at the opposite end you're going to be off. It is simple geometry.