Just remember that the 1.3 is a rule of thumb only. If you sat on the ground for longer than usual you would be screwing yourself out of total time because the Tach doesn't run as fast while at low power settings. Conversly if took off quickly and operated at higher power settings than normal, or went on a long flight you would be logging more time than you actually flew. The 1.3 is something FBOs use for billing but I hesitate to use when logging my time.
To be safe, simply start the stopwatch on your wrist watch when you start the engine, and stop it when you shut down.
If you are trying to fill out your logbook using a bunch of flights with tach time only then I guess go ahead and use the 1.3. Just beware that you have no real way of knowing if you are entering the correct time when using that method. Have fun.