You could also try charging less for ground running time (after the initial brakes-off moment). I know that going to "dry" is complicated but having punters paying £150/hr or whatever while they are waiting at the hold is inviting various kinds of bad behaviour, which I see every time I fly.
There is an easy way round this that a number of schools I know use - you use t.o and touchdown times then just add 5 mins either side to give a standard taxi time, this means that punters are NOT paying a fortune for time at the hold - it also means that they do not rush checks to save time/money.