I beg to differ.
You drain the tank, then fill it a fixed amount at the time - each time you dip it and mark the known fuel level on the diptsick. You get a non-linear scale, but so long as you do it at enough intervals there's no problem at-all, there's no rule that any gauge needs to be linear.
Same applies to sight-tube gauges on tanks such as the PA18.
The same can be applied to any kind of electronic gauge, the Bulldog for example always is calibrated on the aircraft - that's why the twin-tank gauges on that type never have parallel scales.
It's a slightly tedious job, but only needs doing once and is equally valid for float gauges, sight tubes and dipsticks. (Except that the first two you might want on some types, especially taildraggers to do twice - one in the ground attitude and once in the flight attitude, and show two scales).
The problem gauges, which aren't used much on smaller aeroplanes, are capacitance based gauges, because they tend to read differently with different brands of fuel, as fuel make-up is changed with time of year, and with water content in the fuel, which can be storage and conditions dependent. But, even these should read correctly when the tank is empty.
G