Arm Board levelling: is this good enough?

The difference could be the level of machining on the armboard or scale enclosure?

Certainly. I don't know whether a digital inclinometer can really be trusted to be accurate to 0.1°. The armboard was made by Lee Drage at Acoustand, so it should be well made. I chopped it up to repurpose it, and any errors will be related to my mounting it.

More important is that the arm board is in the same plane as the platter surface. If they are both off perfect by the amount shown, you are more than good enough. Even a little more error from true horizontal could be tolerated if the platter has a like error.