Mike, it’s the distance between the proximal plate surfaces. So for you (and a good number of us) it’s 10” from the top of the bottom plate to the bottom of the top plate, or look at it as the “inside” measurement.
Your illustration is indeed a tad ambiguous because the bottom finger of their gauge should be shown extending atop the bottom plate and doesn’t, but intends to show an “inside” distance, or the length of the rubber bag itself.
Joel