The shape is a little bit like a triangular prism when viewed from the side, I.e. looking from centreline of hull to the tank face.the result of this shape is that by the time the fuel is down to say 75mm deep there is less than 10 litres of fuel in the tank, and you should not be trying to run with less.

The photos of gauge earlier in this post show the non-linearity of fuel level.

Certainly a floating arm will give very  poor resistance- fuel remaining relationship. Linear float sensors will be better but not linear.

Calibrating the sensor by filling up from near empty 10 litres at a time and  recording the measurements  is a good solution, but a pain in the neck to implement at a busy fuel wharf.

Has anyone else done a volume vs depth calculation for the H400 tank?

I find the fibreglass bonding the tank to the hull gets in the way of accurate  dimensional measurements, and the subsequent volume calculations are difficult because of the multiple bends in the bottom of the tank. Maybe the tank supplier can tell us? I will ask them.

Using Arduino with look up table, it should be possible to create a quite accurate fuel gauge procedure accurate level is known when stationary.

The sender fitted is a VDO lever arm type