bit of maths is what you need, when you fill up this evening, make a note of how many miles you did and how much you've put in, to work out your actual mpg. then you'll know how far an 80litre tank will take you, and fill up based on mileage.
I gave up using the fuel gauge years ago, I know if i'm averaging at least 25mpg, I can do 400 miles on a tank. frequently driving with the fuel gauge saying '----', but have never actually run out of fuel, even when I think i'm pushing it a bit I can still only get 66 litres in = 14 litres still left in the tank!