Arm Board levelling: is this good enough?

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.

@pinball101 That's a clever thought. I checked and it does read the same whichever way it is facing.

pinball101

"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."

I might just get a digital inclinator

the 0.1 degree shown was when the level was rotated 90 degrees. a down arrow is shown on the left side of the display. That must mean the viewed right corner is a 0.1 degrees high, so put the layers of tape I mentioned on the lower left side 

like pinball said, It should also show the same result after 180 degrees from there, bubble will look the same, but digital might show +0.1, and turn 180 degree show -0.1 confirming the arm board is very very slightly out of level, viewed from the opposite side, the arrow would appear on the right side, indicating it’s viewed right side is -0.1 degrees low.

this one is a bit bigger and includes a bubble level. I’m guessing they pressed the ’hold’ to show -88.8. from where that was taken, rotated 180 degrees would it show +1.2?

A platter, if concave, you can put a small level on the outer edge, and rotate, watch as you rotate. I have had a few platters whereby I needed to remove and reseat on the spindle, sometimes tapping firmly to get them fully seated/flat/level to the surrounding plinth which you should check/level separately in several locations around the platter prior to checking the platter.

If the armboard(s) are separate, it is more important that the armboard (thus bottom of arm post) and platter are level to each other like lewm said.