It should be born in mind that we are allowed to add 5 minutes either side as an arbitrary figure for Taxi time.
Is that an official thing then? If so, where is it quoted from?
I've recently joined a (very newly formed) group based around a DR1050 and they are using brakes off to brakes on to accumulate engine, airframe and maintainance totals. A couple of people I've spoken to (including one who operates a public catagory C of A machine) tells me that you normally count these totals in airbourne time, not brakes-on brakes-off time.