What is the standard for tachometer calibration / Hobb's equivalency?
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.