“Pretty much what ever suits you” might be OK for charging regimes, but care must be taken to comply with the law.
There’s a legal requirement to record an aircraft’s time in service in the MR or its equivalent. There is a definition of time in service. There is no latitude to do anything other than comply with the requirement.
There’s a legal requirement for a pilot to record flight time in the pilot’s logbook. There is a definition of flight time. There is no latitude other than comply with the requirement.