First refueling must be to the full tank

And it must be saved as "First refueling" (not as the "Full"). And it is not counted into the consumption.

Consumption calculating works based on refueling to a full tank.

Further refueling may be either a full tank or partially full. Calculation of the consumption will be made after every refueling to full. For example, if you 3 times refuel partly full tank, if will not affect consumption calculation until the time, when you will close it by the refueling to full.