As I tend to do on these kinds of projects, I ponder solutions well outside the box and often come up with something that can work. My current line of thinking with regard to accurately calculating makeup fuel focuses on an adaptation of the method that the ODBII type systems use. Those systems measure and total the time that an injector is open. If you know the flow rate of the injector and the duration it is open, you have fuel usage. Instead of measuring fuel flow with an impeller and a hall sensor, or a ball in a race and an optical sensor, I'm wondering about the feasibility of using a very high flow injector (100#/hour range) tied to a pressure sensor in the surge tank. Under this scenario, the Arduino monitor the pressure and would turn on an injector in the surge tank whenever the pressure was at a low limit, say 3#. Since injectors are designed to turn on and off quickly you can shoot for near-zero hysteresis and therefore get to decent instantaneous fuel usage. The Arduino would measure the time that the injector is on and get fuel used based on the time. There are some difficulties with this setup that would arise in cold temperatures based on the change in viscosity of the fuel.

A similar, and maybe better option would be to use a low pressure, positive (and fixed volume) displacement pump of some sort that would enable just counting "clicks".

Just knocking around ideas that might lead to a solution down the road.

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