How critical is the exact placement of the tonearm

VERY!!

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! (I don't have an RS-A1, and these may not be the precise instructions, but they are quite casual about placement.) By all accounts and based on my one listening experience, the thing sounds just great. Go figure.

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.