I bet he hates it when he does that over and over again!
PNR = (Flight Fuel - Extra Fuel Burnt on Climb) / (SGR Home + SGR Continue)
That works fine on the ground but after you're airborne, pick a point eg current position, note fuel and distance from Home and run your PNR forward from that point, reducing your "Flight Fuel"/Safe Endurance by the fuel you need to get from your current position back to home, minus any fuel you save on the descent.