Arm Board levelling: is this good enough?

The most important thing is that it sounds good.  Remember your records themselves are not all perfectly flat either.  Being off by such a small amount is insignificant given the overall vagaries that are inherent in vinyl playback mechanisms overall.

The most important thing is that it sounds good. 

Really.

If the armband is not level the arm is constrained.

You worry about 0.1g tracking - an out of level arm board has a large impact on the bearings and arm motion, not to mention the unbalanced forces on the cantilever.

What do you do when the cantilever is bent or stylus worn more on one side - do you turn it around to wear out the other side ????

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.