So how are people using a level like this to accurately measure either the side-to-side or front-to-back level?

Levels (digital or "bubble") are only going to be correct if they are "squared" in the opposite direction that is being measured.

People here always say that you should measure on the playfield (not on the glass). But if you put the level on the playfield, you need some way of being certain that you are not slightly angled, because if you are, the readings will be off.

Maybe if you are lucky, there is some straight-lined playfield artwork that you can key off of, but often there isn't.

If you measure with the glass on, you can use the side rails and the lockdown bar as straight edges, but then you are assuming that the playfield is "in line" with the glass.

Seems like either way has potential issues.

Oh, is there some sort of best way to clean a playfield...?

This picture is exactly what I was talking about. The article uses this picture to demonstrate a level playfield.

However, the problem is that even though the bubble is centered, the level itself is obviously not squared - which means the playfield is NOT actually level.

The same problem applies to digital levels and smartphones as well.

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