I just refill mine when the amber warning light comes on, divide the mileage covered by the litres put in, mutliply by 85 and often realise that I could have motored another 80+ miles before I would have been really empty.
On a bad day it's 425 miles per tank full, but on a good day, and with easy gentle soft driving, or constant 80mph driving, the tank has returned 500+ mptank
Malc