The fuel level sensor that comes with the boat is pretty inaccurate. I'd only trust it to tell me the tank is either full or empty and don't trust it for anything in between. The problem is the sensor is linear but the tank is irregular so the readings on the guage in the cabin are just plain wrong except when it's full or empty.
I purchased a Lowrance NMEA fluid level sensor. This attaches to the sender on the fuel tank and your NMEA bus. On my 439, there is a NMEA point just in front of the tank on the port side where the autopilot computer is mounted. You will need a sim-c to NMEA convertor cable and then it's pretty easy to install. I crimped some female spade connectors to the red and black wires, disconnect the stock wires from a plug on top of the tank, attach the new spades instead of boom, it's installed.
It gets picked up automatically by the chart plotter and I could configure it. You give it a name, provide the tank capacity and then you can look at the "data".
My tank is about 7/8 full, the sensor shows 33%. The problem is the sensor is configured for a US sensor which is 240 ohms empty to 33 ohms full. The sensor is not a US sensor, it's a European one. They decided to do 10 ohms empty, 180 ohms full. So, 33% means 66.1 ohms (180-10)*.33+10. This translates to 33/(240-33) or 85% full with a US sender which is about right.
I just need to calibrate it which is a pain to do and it requires pumping out the diesel and then filling it carefully and clicking on a "calibrate" button on the plotter at various fill points and then it's pretty accurate. So, I removed it for now until I can do the calibration, back to engine hour counting...