If you're going to be flying jumpers then you're likely to be using minimal fuel. If it doesn't have a reliable dipstick then make your own dipstick!
Get a stick or old ruler. Run a tank dry during a flight (but make sure you departed with plenty of fuel in the other tank). Dip the 'dry' tank before refueling. Mark any fuel level that shows as 'zero' (it's unusable fuel, for practical purposes). Add a fixed amount of fuel eg 20 litres and dip again. Mark the fuel level. Repeat. Preferably repeat until the tank is full but you may not have the opportunity due to weight limits for subsequent skydive flights. At least do it enough to give 2 or 3 drop runs.
If the fueling place isn't level then you may need to have more than one set of calibration marks - one set for refueling, and another for when the plane is in its usual parking spot.