There's tiny bug in ETS2 plugin: It doesn't load the fuel tank capacity correctly. This affects how settings work. Instead of fuel percent trigger, it's triggering on amount of liters left in the tank. Setting the option "Fuel perc." to 100 will make the "Fuel low" event in Fanaleds to trigger if there's less than 100 liters left in the tank.
So far, the issue wasn't fixed yet. ETS2 Telemetry has the fuel capacity variable. If you divide remaining fuel(in liters) by the capacity(also in liters) you'll get value ranging from 0-1. When that value is multiplied by 100, you'll get the exact percentage of fuel remaining.