As a former fuel system engineer in the automotive industry that actually did a great deal of work on fuel gauge calibration, I can tell you that fuel gauges, on automobiles, are notoriously inaccurate. One factor that needs to be accounted for that there is always a certain amount of fuel that is unreachable in the tank. Personally, I assume that amount is one gallon. Another consideration is that boats heel and pitch from wave action etc. This causes the fuel pickup to potentially suck in air when the fuel level is low. I recommend a more conservatism in your estimations when traveling on bumpy water.
To calibrate my fuel gauge, I carefully log my engine hours, and the fuel needed to fill the tank. I do this understanding that each time the tank is "filled" will be somewhat different. Yanmar publishes fuel usage vs. RPMs for its various engines. Hopefully your manufacturer does as well. I can compare this to the calculations I make using my fuel log. This gives me confidence my estimations.
Here is an example:
My fuel tank is 27 gallons, subtracting 1 unreachable gallon is 26 gallons. If it takes 10 gallons to fill my tank, from my last fill, when the gauge reads 1/2, then, 26 - 10 = 16 gallons of usable fuel remain in my tank. According to my log I put 12.5 hours on my engine. I calculate that I used 10 / 12.5 = 0.8 gallons per hour. I cruise at 3000 RPM. The published curve for my engine shows the same fuel usage per hour. This gives me the confidence that I can travel for 16 / 0.8 = 20 hours on flat water or 120 NM at 6 knots when my gauge reads half full. If there are rough conditions, I would half that.
