Dude~ has introduced a very important point which we should have included much earlier in the discussions. i.e. when does the charging time start and end?
I know of the following arrangements used by different owners:
Brakes-off to Brakes-on.
Hobbs meter which starts when master switch moved.
Hobbs meter with oil pressure switch which starts when engine started.
Hours meter in engine tacho which records 1:1 at cruise rpm and therefore much slower when warming up engine, doing checks, and taxying.
Take-off to land plus a fixed time (often ten minutes)
Take-off to land, i.e. just airborne time.
It always seems completely wrong to me to charge the same hourly rate while I am warming up an engine, or checking and setting instruments, or waiting at the hold. Seems crazy to encourage hirers to skimp warming a cold engine up properly, and to rush the checks and taxi, just because the owners want to maximise revenue.
As per an earlier post, our group charges take-off to land time, so there is no pressure to rush the warm-up, the checks, or the taxying, and if ATC want you to wait ten minutes at a holding point, there is no real financial penalty.