So the first question you answer about the tank size is only there to set an appropriate increment. It doesn't enforce the "top" of the fill up. That's determined on the fly as you calibrate. So, tell it you have a 50 liter tank on that step, and then it will ask you for 5 liter increments instead of 10. if you go lower to say 25 liters, you can even do 2 liter increments 0 but it will take forever of course.
