My first 100hrs were in an aircraft that did not have a hobbs meter (Cessna 172N). Here's what I know, and what I'd recommend.
The tach is calibrated for a certain engine RPM, very close to full power. I was told it was something like 2300RPM's. Anyway, if you are taxing at saying 1150RPM's, you could taxi for .2 hours before .1 would turn over on the tach.
If you were doing a training flight (takeoffs, landings, stalls, other maneuvers), the tach was much less than the actual time. Therefore I would recommend looking at what time you start up the engine (recording it), and then recording the time at which you shut down your engine for those flights. I have gotten 1.1 hours of flying for .6 hours of tach!
For cross country flights, I found that .8 hours of tach generally equaled 1.0 hours of flight time. This seemed pretty accurate for me. If you are running your engine at full power, your's would probably be about .9 hours tach for 1.0 hours of flight.
The most accurate way is to obviously wear a watch. But if you are doing a cross country at the power settings you say you fly at, it is between .9 and 1.0 hours of tach for 1.0 hours of flight. .9 looks pretty consistent with your results. Hope this helps.