There is a few things you can do. First, reset your odometer everytime you fill up, then just run it till the needle is pretty much spot on the empty line. Then take note how many litres it takes to fill it. Then you can work out your average economy and then work out how much further you could drive based off how much is generally left in the tank when the needle is on the empty line. This method will give you an accurate way to calculate how much fuel you've used and how much is left down to a few litres. Much more accurately than any fuel gauge as they can work in funny ways depending on tank shape. For example, I've had 2 cars where I could drive close to 150kms before the gauge moved from full, yet my current car will be down to 3/4 after that distance, but yet the last 1/4 (from 1/4 to the empty line) I can do close to 300kms.
But I don't see what the problem is really. Just fill up when you want/need to. It makes difference to whether you fill up with 20l left or 10l left. Your economy will be the same, and in the long run it still costs the same.