Now after 6 fuelings I can give you some results.
I have put 300,48 litres to the tank.
And MPGuino says I have used 298,44 Litres.
The total error is then 0,68%, which is not bad at all. I must say I'm very pleased with the results and finally I can trust that MPGuino in my "Plastic spaceship" is accurate.
Well, ofcourse I can do yet a little fine tuning to minimize the error a bit more and.... AAARRGH! why can't I just be happy and let it be?