Thanks for the replies!
Here goes my hope of them refunding it at their own refueling rate (which is about 2.5x the cost of fuel at a gas station)
Will return it 3/4 full then. Thanks to the mileage and consumption calculator built into most cars, it's actually quite easy now - you calculate exactly how much fuel you have consumed based on the mileage driven and the average fuel consumption shown on the dashboard, then add the estimated fuel amount for the way back to station (again, based on actual numbers) and put exactly that amount of fuel into the tank. This ensures the fuel gauge will be at the same position when returning as it was when picking up the car.