Fuel: gosh, why not use - for basic PPL training - an aircraft with a Rotax 912 and allowed to fly on unleaded.
Take a C42 - fuel burn about 12l an hour at £1.40 pl = £16.80.
Even if fuel goes up to £1.68 a litre that would add £3.36 an hour to the costs.
If you can't absorb that, your profit margins are too thin.
I wouldn't spend my time adjusting daily rates for such small sums: far better to spend the time campaigning for a workable system! Or the ability to use ordinary unleaded - it never seems to cause big problems in microlight land.
Jock, I can see your point for twins etc, though.