You can only input the data once you've driven so many miles AND filled up with fuel, otherwise how do you know how much fuel you've used.
An example scenario, full tank, start trip. Decide to fill up again whether your tanks a quarter full, half full or even three quarters, it matter not. Brim the tank and make note of both miles driven, and fuel used. Input that data, fuelly does the rest. It's very simple. A partial fuel up is only if you don't fill the tank completely.
|