Tach time seems like a good way to go for high tows, but I won't make any suggestions about what rate to charge, as situations differ. However, for the "normal" tow, which can be low, as for repeated pattern practice, the typical Hobbs meter that only displays in tenths of an hour, a tow could be 0.1 hour or 0.2 hour or even 0.3 hour, depending on how the numbers click over. Tach based charges also have the effect of charging someone for time spent waiting for traffic to clear, slow hook ups and situations where the tow plane is running with no gliders ready, but not so much of a delay that a shutdown and restart is warranted.

Plus, I doubt that the tow pilot really wants to be that much of a bookkeeper. Reading the altimeter is much easier than checking the tiny numbers on the Hobbs, which is often not in an easily viewed place on the instrument panel.

And let's not forget that tow pilots also differ. We had one who ran B-52 sized patterns that added excessive delays to busy day operations.