Firstly you are required to log brakes-off to brakes-on. Hobbs time logging is not correct or legal because you are logging time the engine is running but the aeroplane either hasn't started moving yet, or has stopped. If you are being charged by Hobbs, live with it, you still can't log all of that time.
Secondly, legally a flight is from brakes off WITH THE INTENTION OF FLIGHT. However, in reality if the flight didn't happen, most people won't, and nobody really expects you to.
G