How critical is the exact placement of the tonearm

A drill bit isn't used to control center to center positioning of a pair of holes (or an array) when close tolerances are required. It is simply used to clear material out of the way in quick fashion for a more accurate method of machining. I wouldn't trust using a drill bit to maintain center to center positioning for tolerances tighter than 0.010" (.25mm) I'm not saying another machinist wouldn't TRY to drill 2 holes within that tolerance but I wouldn't. Sometimes you get lucky but it isn't worth the risk. Do it right the first time.

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).