CS,
I take the flight time(take off to touch down) from the flying log and add 5 mins to each tow for taxi time. This seems to work fairly well for me. I've compared my timings against the engine hours and its pretty much spot on. Everyone has their own method though, some go for 10 mins to 2000ft and add 5 mins to every 1000ft thereafter. Depends what your flying too, the likes of a pawnee climbs much faster than most. Our chippie is a 180 but with a 2 seater with full water ballast behind you, its a long time on tow! Much nicer to fly than the pawnee though.