Was always a bitch, 20 years ago, to do a crosscheck of the fuel in a tank of an ATR in Fiji when a gauge was u/s. There were two magnetic level indicators mounted in each wing through the lower skin, and tables allowed the readings to be converted into units of fuel mass with corrections made for aircraft attitude (not being on a perfectly level surface - to small limits), and fuel density. The actual magnetic indicator markings were in cms. of fuel in the tank.