This app calculates fuel endurance based on the fuel available on board and the fuel flow rate. Adjust the fuel flow multiplier as needed — e.g., use 1 for one engine, 2 for two engines, etc. If reserve fuel is entered, it will be subtracted from the fuel on board for endurance calculations. The app also calculates the PNR (Point of No Return), which indicates how far you can fly before you no longer have enough fuel to return to your starting point. PNR is determined using the entered TAS, track, and wind data. GS Out is the ground speed on the outbound leg toward the PNR, while GS Home is the return leg ground speed, both adjusted for wind.

Functional Demo: https://www.theairlinepilots.com/apps/fuel-endurance-pnr.php