Every one is a little different, and your best bet is to calibrate it yourself. Carry some gas, run out (safely), and then you know. Watch out for running the fuel pump dry, since it's a risk.
I have the 22 gallon tank, and when absolutely run flat out, I pumped in 21.6 gallons. After the light came on, I had driven about 20 miles offroad and 5 miles on pavement, and I usually get 12 MPG under those conditions, so I assume I had about 2 gallons left when the light came on. If you're on pavement alone, and get better than 12 MPG, you should go farther. But no guarantees unless you test your own specific vehicle.
There are a few things you can try to find out. As soon as the light goes on, go and fill-up. Subtract the gal. put in from what ever the capacity of the tank is. ( 22 gallons?) Or keep a gas can in the back and drive until it is empty. Do you have the D.I.C? Check the Range setting. There is about three gallons left when the light goes on IIRC.
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