I can't think of any occasion when a Hobbs meter is correct for a pilot's logbook.
FAA (
FAR Part 1 Sec. 1.1 effective as of 10/12/2012) is pretty clear that:-
Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or
So it's neither "oil pressure time" (which would be over-logging) nor take-off to landing (which would be under-logging). It's what is often called "blocks time".
We use basically the same definition here in Britain, and I'm continuously astounded at the number of intelligent adults, capable of passing a pilots licence, who nonetheless are apparently incapable of writing down and logging what time the aeroplane starts moving, and what time it stops.
And of-course this is not a "generally accepted norm" it's a rule. It happens in this case that UK and US have the same rule - at-least for civil aviation and for single pilot operations.
A tacho, which is generally calibrated to max RPM, will usually underread compared to a Hobbs or to blocks time. 0.4/0.6 isn't that unusual a difference - if you're normally getting them very similar, then presumably you do very short taxiing, little or no time stationary with the engine running before and after a flight, and generally fly close to max RPM ? (Or, I suppose, the tacho is calibrated to less than max RPM.)