the correct method is a manual log of time from brakes off to brakes on
True. But for all practical purposes, how much time is there between engine start and brakes off, and how much time between brakes on and engine off? If it's more than a minute in total you might want to take a careful look at your checklist and ask yourself why you are burning all that expensive avgas, without using it for airframe propulsion somehow.
Generally it's less than a minute. Maybe two if you're really slow. Since you're rounding to the nearest five or six minutes (depending on the way you log: minutes vs. decimal hours) that time is rounded away anyway.
So if your aircraft has a hobbs meter, and the hobbs meter is wired to engine on/engine off, you might as well use that as your basis to log flight time. (But note that a hobbs can be wired to anything. At our club we've got a few aircraft that have a hobbs wired into the pitot/static system, and they only log time when the aircraft is doing 30+ knots, i.e. flight time.)