If you're paying for it wet then you've already paid for the fuel in the hourly rate. In which case you're best served by going like the clappers to get there & back as quickly as you can.
Fly at the Full Throttle Height (ahem....altitude) for max continuous power in nil wind. If wind then work out the difference in TAS gained or lost by climbing or descending vs. the beneficial effect of more tailwind or less headwind.
If paying for it dry ie you buy all the fuel, then you need to compare the fuel consumption at different power settings vs the flight time to see how fuel consumption costs changes wrt to the cost for the aircraft using the same speeds.