Have to say i would have thought you would have jumped in straight away with the time on the ground. Is it fair you pay for 20 minutes holding on the ground at the flying rate--I do not think so especially as the airframe and engine hours are taken from take off to landing.
Ah, the old "how to charge for flying" discussion. Very simple. If you rent a plane from a commercial outfit, you can be sure they will set the rates so that they maximise profits without turning people away. They're a business, after all.
As far as a club is concerned, with no profit motive, what they should do is tally up all costs for an airplane, meaning fuel, maintenance, hangarage, insurance, depreciation, whatever, and divide that by whatever convenient number can be easily and reliably measured from the instruments in the airplane. The pilot then has to pay whatever the instrument says he has used.
There are several variations. There's hobbs meters, which can be wired to the electric master switch, to the engine master switch (in case of FADEC), to an oil pressure sense switch, to a pitot pressure sense switch and probably a few other places as well. There's also places that use the tacho, and places that use the honour system requiring you to accurately report engine on, off-blocks, take-off, landing, on-blocks or engine off times.
But at the end of the fiscal year, the exact method of deriving your flying time is divided out. So it really doesn't matter all that much. A system where you only pay for actual time in the air will have higher hourly rates than a system based a hobbs meter, which runs all the time the engine is on, all other factors being equal.
The only real difference is whether the system favours long flights over short flights or not, and whether the system encourages flying at a "proper" cruise speed, properly leaned, or favours flying at the highest power setting that's allowed continuously.
It seems to me that using the tacho as basis for charging flight time is the best compromise, although this doesn't yet encourage proper leaning. Because of that, long distance, high-altitude (typically IFR) flights are comparatively expensive for the costs that these flights incur.