dont worry about it mate, you'll get used to it.. keep it filled to the brim and fill at half way... there was a post on a forum somewhere about how once you go buy half a tank the other half of your tank is filled with petrol vapour which you wont get to use, so if u minimise this then you'll get better miles for your money. ie if you run your car low you have about 60-70 litres of petrol vapour in your tank, dont know how much this when in its liquid state, but as soon as you open your fuel tank it all escapes...
also buy fuel early in the morning when ambient air temps have been at there lowest and the fuel is in its most dense state...
cant remember where i read it but there does seem some logic in it