212man, don't give up your flying job to be a math teacher! If you are burning fuel and going nowhere with zero groundspeed, you divide the fuel flow by zero and get infinite SGR, which means infinite fuel to go one mile. The "formula"makes sense.
BTW, this is not rocket science, guys. Take the fuel flow and divide by the ground speed. The pounds/hour divided by the miles/hour gives you pounds/mile, so you always know how far your gas will take you, a much better piece of knowledge that how many hours you have left.
The S-92 goes one better, it has the fuel gauge and the fuel flow gauge on the same scale, so the arrow of lbs/hr fuel burn points at the same bar that fills in the fuel amount. If the burn rate is half way up the fuel gauge, you have 2 hours gas remaining. If the burn rate arrow lines up with the fuel amount, you have 1 hour fuel remaining. (for example, burn 1000 pounds per hour, with 1000 pound of fuel remaining).