Arm Board levelling: is this good enough?

It's not that bad; AS can be exactly correct at two points across the surface of the LP, because the magnitude of the skating force describes a hyperbole, more or less (a U-shaped curve), and the AS can be a constant, both in terms of grams of force. A straight line (AS as a constant force) can intersect a U-shape at two points.

The simple test of accuracy of any level is to take a reading then rotate the level 180 degrees and compare that reading to the original reading.   If it reads the same when rotated it is accurate.  

Nice setup!!!!!!

I use a variety of levels, much easier to convince myself all is well.

you moved into a world of precision I purposely avoid.

The width of the tool looks similar to the width of the tonearm base.

You could test the tool against other levels, and if you have confidence in the tool, then I would put a few layers of blue painter's tape under the 'low' (hah) edge until 0.1 is 0.0

then, put that many layers of blue painter's tape under that edge of the tonearm, so the arm presumably gets to be 0.00

I thought platters were designed in a slightly concave manner?   I always ensure my plinth is level on my basis tt.  Did I dream that?

It's nice if the plinth is level, but what is most important is that the platter and tonearm mount board are in the same plane AND level. You make a good point about the slight concave-ness of some platters. That would be confounding if present.