I've heard that 1 tach hour = 1 Hobb's hour at maximum RPM. I've also heard that tach equivalency is at "cruise" RPM, although that seems vague.
I believe I've also heard that the accuracy of tachometers is regulated (I think to within 50 RPM).
Because engine time, and often TTSN, is measured by tach hour, there surely must be a strict standard for each aircraft for what RPM the tach counter should be set to.
What is the standard for tachometer calibration? Max RPM? Cruise RPM? Something else?