These readings are boat and prop specific.
I usually cruise at 5 inches which gives me around 2100 RPM. My max RPM at 0 inches is around 2300 or so.
In flat water, if these reading change for the worse, i.e. less RPM at a certain vacuum, I know the bottom is getting dirty. Right now I *really* need to clean the bottom and full throttle (0 inches) is only getting me 2000 RPM or so and 5 knots
I'll fix that as soon as the water looks safe to swim in.