I'm confused as to why you only log time in the air vs taxi time. Your flight time starts the second you start rolling towards a runway.
Title 14 1.1 says:
Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing.
any time that hobbs meter is ticking of minutes is time you can log in your book.