Most places I am aware of charge simply by Hobbs Time. The general
rule of thumb arround here is that on an average the Tach time will be
80% of the hobbs time. Most businesses do it this way because Hobbs
time generates more revenue for the airplane per maintenanence hour.

Example: If they charge $100 Hobbs for the airplane.
They get about $120 for every Tach Hour. Since they do maintenance off
of Tach time they get an extra $2000 for every 100 hours put on the
airplane over charging Tach time.

I do understand why clubs might prefer to charge by Tach, but they
usually do so by charging a higher rate than if they were charging by
hobbs. Charging by Tach encourages the pilots to throttle back some
which can make the engine last longer.

It makes sense to me to charge by Hobbs on constant speed props
because charging by Tach encourages running High MFP and Low RPM. Low
RPM is ok but need to keep the MFP in line with it as well.

With the fixed pitch prop, the roller coaster approach is probably the
least expensive. This is simlar to what motor gliders do for cross
country. 2nd would probably be to simply run at the best economy
cruise.

Brian