I kind of calculate from the other end...at 300 miles it usually glows on...which works out, at my mpg, around 4-4.5 gallons left. Thus, I can drive safely for about 50 miles, before I start getting REALLY nervous. Then, hope for a gas station.
At first I did not believe the 5 gallon reserver as stated in the FSM.
A few months ago I drove 64 miles with the light on.
I was near home. I get about 14 mpg with 285's so thats pretty much as far as I will press it.