Arm Board levelling: is this good enough?

That is why I regard the arm board being level with respect to azimuth (it is) as more important because that will cause uneven wear and likely not sound good. The 0.1° error is the armboard tilted sightly down towards the table which if it has any effect is going to make the arm want to move towards the middle of the side of a record, in effect slightly increasing skating force for the first few tracks, then decreasing it for the last few tracks. Well, I already have to accept that anti-skating force is only correct at one point on a side, and as Bill says, a warp in the record will cause far more variation from ideal.

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?