If I recall correctly correctly the vacuum suction gauge in the TB10 has a scale from 3 to 6, with the green range being 4 to 5 (I might slightly off on those numbers though).
It's clearly marked as showing inHg, but unlike the manifold gauge it's on the lower stop when the engine isn't running (rather than the upper stop if, as the manifold gauge, trying to reach ambient pressure). I haven't really paid attention to it's behavior in the aircraft (only checking that it's in the green), and I just looked at it in Xplane right now which might be wrong of course.
Is it showing the negative difference to ambient pressure? Or the difference in pressure before and after the vacuum pump (which may or may not be the same thing)? Or something else completely? :)