The only way to know for sure is to run it dry under safe conditions (within a couple miles of a gas station, and/or carrying a few gallons of spare gas in a can).
For me, I used to get at least 70 miles after start of blink, and would fill close to 12 gallons. Lately though, I've used 50 miles after blink, and could only fill 11 gallons. So either the tank 'shrunk' in which I can only get a little over 11 gallons right now, or my guage shifted and now it starts blinking early, so I still have at least one gallon and at least another 50 miles left. Again the only way I could know for sure is to run it dry.
I did unintentionally and absent-mindedly run out of gas, and I think I got 83 miles from start of blink, at about 52 MPG average, and I think the tank took something like 12.3 gallons, accounting for the 3 gallons the roadservice provided.