From the NH manual: "Proof Meter - records the hours and portions of hours your tractor has been operated based on an average engine speed of 1833 RPM. Engine speeds below 1833 RPM accumulate eninge hours at a slower rate than clock hours. Engine speeds above 1833 RPM accumulate engine hours faster than clock hours."
I tend to run most of the time around 1800 RPM, so the hour meter should be fairly close. Since we're still developing the property, we don't do much mowing, which is around 2400 RPM or so to get 540 PTO RPM on the TC18. I do boost the RPM's when I need some extra "oomph". Just today, I was stretching about 400' of pasture fence, and kicking it up to about 2600 let me pull it another 6" or so - then quick on the brake - that fence is really tight! I don't run very often below 1800, so the hour meter is probably running a little "fast" - my guess is about 10% - so my clock hours are probably around 180. That's only about 45/month, or a little more than 11/week. That's not so much. It gets used for something almost every day. If you count the hours it was useful, like today, sitting still and holding the fence tight for several hours while we stapled it all, the time would be much higher.