Tacho time does not relate to flying hours,where I think .80=1 hour engine.
Tacho time is related to the actual RPMs. At a specific RPM (depends on the engine installation and tacho calibration), 1 tacho hour = 1 wall clock hour. Halve that RPM and 1 tacho hour = 2 wall clock hours.
It makes sense to charge based on tacho hours since it's got a strong relation to engine wear/TBO and fuel consumption, which are the two most costly variable costs of an aircraft.
Logging time for logbook purposes is formally from the moment the aircraft moves under its own power for the purpose of taking off (so not for the purpose of taxiing to the fuel bowser) to the moment it comes to its final rest after landing.
also how are the "hours" hours worked out. 0.8 is 50 minutes I take it, .5 = 30 mins 55mins = 0.9hours?
Just round up/down to the nearest decimal, yes. Or record everything in hours and minutes. Make a decision and stick to it.