Tank contents of current Cooper S

Hello together Since November I own a new Cooper S. Now I would like to know how much gasoline fits into the tank. Lt. information is 44l, but I think this fits in much more. Reason for my question is that I filled the car at the petrol station until switching off. (range 630km ) Then I pressed the flap in the tank socket with a pen and it went in again 3l. Then I drove home ( 1.5 km ) and then still with a Kanist Here I say and write I can refuel 15 l. Thus the range should be almost 800 km. With my other two BMWs that is not so. If here at the gas station the pump shuts down, then is full…… And with the mini just not. That’s why my question. Stix3