I don't get this "thin" bit comment/s
Flight planning is not a guess it is a formula and that formula has built in factors for regulatory requirements.
At the end of the calculation "x" fuel is required or the flight can not happen - X is not a thin number, it is the number that should have you at your intended destination with required reserves.
Should a variable be greater than expected during the flight in the flight plan, then the options considered in the flight planning may need to be used such (as a divert for fuel). That does not make it "thin" it is in the plan not to run out of fuel.
I expect that a divert for fuel would make the profit margin thin, but that is not one of the factors in the flight planning formula.
* It could draw a few punters from the East Coast that have family in Perth but don't want all their holiday there but want to also head to Europe.