E120 Fuel Used

I have Yamaha F250's connected to my E120 via a NMEA2k network. I get all of the engine data on my MFD, but can't figure out how to get the "fuel used" value to work. I get fuel burn, so presumably fuel used would simply be a math function storing the fuel burned values.

Can you tell me how to get this feature to work properly? I would much rather use this feature than a tank level sensor in my tanks due to their shape.

Thank you!