Mine takes 14.9 gallons and says full when it is full, when it has about 2-3 gallons left, the light comes on. Plenty of times it has come on immediately when I have left home for work and I make it back home, which is 52 miles round trip. The light isn't exact, it varies somewhat, even goes off after coming on, when it comes on and stays on, I know I am good for 50 miles, I have intentionally ran it out to see how much, my conclusion was 75 miles is beginning to push my luck. So I am assuming ~2.5 gallons when light comes on.
I don't know about the RAV, but in my Pontiac there is a variable resistor that controls accuracy of the gauge. Not easy to get to.