You must think of this gauge as an indicator of the "level of fuel" in your tank, "not the volume of fuel". This gauge has limited use for me in the real world. I use my ScanGaugeII, to display 'Distance to Empty' and it provides a much more accurate and useful number.

As well, it is generally a good rule of thumb to not let your fuel tank get too close to empty. The fuel pump is located inside the tank, and is cooled by the gasoline that surrounds it. You risk burning out your fuel pump, if it overheats, and it can be expensive to replace.

 

I came to the same conclusion of 50 miles differently.
Light came on and I drove 20 miles more (highway) - when I filled it up it took 9.6 gallons. At 36mpg highway (for that tank of gas) it means that 50 miles is about what I had left when the light came on - highway. In city it would be less.