In theory if you used Tacho time for flight and duty compliance you would be exceeding your actual flight time limits if you were calculating to the maximum without an adjustment. Hence why there is a strict method for calculating flight time. Tacho 'time' is misleading as it does not measure hours and minutes, its a counter that measures engine usage. It does relate about 80% to real time, depending on things like length of flight and taxi, but obviously the lower the average RPM used for a particular flight the slower the counter will tick over. VDO/HOBBS would be the closest approximation to the legal requirement. For private operations, who cares, if you are building hours for your commercial or experience in general you will shaft yourself out of approximately 20% flight time.