As someone who supplies aircraft to flying clubs I had to change from charging from take off to landing according to the entry's made by pilots in the aircraft log, however it became clear that a small number of the people had difficulty in getting the numbers right and always seemed to find tailwinds.
I changed to charging using the tachometer times, I was very careful to set the tachometer rate at one that would reflect the rate for take off to landing rate and not disadvantage those who had previously accurately recorded flying times.