I used to regularly get at least 120 from a tank before the light came on, and would then get another 20 miles before I would fill it up - I never let it get too low to avoid sucking up any crap that might be in the bottom of the tank .
There is no 'reserve as such' - the fuel light flashes to let you know it is getting low, it is up to you when you fill it up again!
The only way to find out how far you can get will be to let it run out (suggest strapping a fuel can to the back first
)
Good luck!