One also needs clear rules on things like duty drawback, otherwise you will get 1 or 2 members always flying over to LTQ and pocketing the drawback. The drawbacks should go back to the group fund, but that is not perfect accounting either...
Why?
If the hourly rate is a "wet" rate and you acquire fuel elsewhere, the group should deduct the cost of that fuel from your bill. And I think the deduction should be based on the fuel cost at the home base, not on the actual purchase price and/or whether or not a duty drawback can be done.
That way you reward individual members if they shop around for fuel (they pocket the difference themselves, after all), but at the same time you're keeping the work for the treasurer easy - all s/he needs to know is the amount of fuel somebody uplifted.