I average about 18-20 miles, after the last light has gone out. At that point, the amount of gas that I can fill up is just about 19 gal, usually around 18.7-18.8gal. The 18-20 miles that I drove, I did this 2 or 3 times, was a mix of highway and city driving, but more on the city side. In that situation, I usually average around 18-20mpg, as I've been watching this stuff pretty carefully.
CONCLUSION: There is approx. 1, maybe close to 1.5gal left in the tank once all lights are out. Also, seeing how its a 19 gal tank, subtract the 1 or 1.5 gal above (from conclusion) that leaves say 17.5 gal that are spread out over 16 lights...or approx. 1.1 gal per light.
Hope this helps