EASA gives the definition of Flight Time - but does not indicate to what
degree of accuracy this must be recorded.
To the nearest minute, to the nearest five minutes and to the nearest
six minutes (1/10th of an hour) are, probably the most common.
I cannot, therefore, see any problem recording Flight Time from either
actual "Brakes Off to Brakes On" or as a
notional "Brakes Off to Brakes On"
based on 5 (or 6) minutes added before the Take Off and after the
Landing times.
Personally I like "Take Off to Landing + 10" as, for aircraft charged by
Flight Time, it removes the financial incentive to rush the checks prior
to departure.
For the OP:
E.g
Departure: 15:30 / Arrival: 16:00 / Total Time Of Flight: 0.7
I would assume this will be picked up, and queried, by the CAA as
Flight Time
must equal Arrival Time minus Departure Time.