Custom and practice, from 30+ years in this game, and professional qualifications in most places.
- In the USA flying logbooks are engine start to engine stop, maintenance records are on tacho
- In the UK for microlights only, engine logbooks are from engine start to engine stop, flying logbooks and airframe logbooks are on brakes-off to brakes-on
- In the UK and Europe for civil aeroplanes, flying logbooks are brakes-off to brakes-on, both engine and airframe logbooks are airborne time, except that a few follow the American practice of using tacho.
- In the UK for military aeroplanes, both flying logbooks and maintenance records are for airborne time.
I think it is fair to say that...
subtract 10 mins per flight for eng logbook
Is wrong absolutely everywhere.
It would also be fair to say that when you burrow down it can be hard to find definitive legal documents for any of the above, but generally the definitive answers are somewhere in the maintenance schedule authorising documents.
G