Hi Stig,
My car (same as yours) is just the same – the gauge reads too low and never goes to “full” when the tank is filled to the brim – rather than messing around, why not live with it? You know the size of the tank, you know the fuel consumption (typically and in Imperial units – 15 gall tank, 20 mpg = 300 ml range). I’ve driven about 120km with the low level warning on the dash but keep a petrol can in the boot just in case!