What is the standard for tachometer calibration / Hobb's equivalency? 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?
Hobbs time is most useful for tracking log time (and billable time in rentals.)

Tach time is simply a counter of revolutions of the engine, displayed as "hours."  Calibration is basically at normal cruise RPM, which can vary for different engine/prop/desired speed.  The tach counter is approximate at best, but it is what we have.