</font><font color="blue" class="small">( Is there any standard for proofmeters or usage meters or revolution meters or whatever name they have? )</font>
Well, yes. The standard is that meter hours will match clock hours *at rated PTO speed*, that is, at the ticmark on the tach which gives 540 rpm out the PTO.
On the old tractors the meter is driven by a rotating cable like a speedometer and the 'hours' are actually a count of revolutions turned. This is good for planning maintenance, but not helpful for billing by the hour unless you really operate at PTO speed constantly.