So my boat has a Trapezoidal fuel tank. The sight gauge is quite difficult to read so I decided to make a simple dipstick.
Obviously if you dip a tank with a triangular cross section a half full tank is not halfway up the dipstick. So I was just getting my thinking cap on to do the maths but of course I can just be lazy. Everything is on the net.
Someone has already solved the problem and written a handy on-line calculator here
I have to confess I am feeling a bit lazy though !