I have a 4 sided length of wood marked as follows, 2 sides for the mains and aux tanks when the plane has the Tundra Tires, and the other 2 sides when I've got the little wheels on, marked up in 10 litre increments starting at the 30 litre level.
After i get done flying, I mentally calculate how much fuel should be left based on the type of flying i was doing i.e. staying low or having a long climb at high power then measure it to see how close I am. Now I know that is may be unnecessary, but the joy i get when the calculations are within 5 litres really puts a smile on my face and makes me feel like a right smarty pants.
Sad b*&tard huh !!!