The best I have achieved is 150 miles on the 3/4 mark , 250 on the 1/2 and 350 on the 1/4 with the 8lt light coming on at around 415 miles.
Mind you all long journeys on single carriageway A roads with little or no driving in traffic, and I was trying to squeeze 5 return trips to my place of work without filling up. (because I was leaving very early and getting home late)
in general anything over 350 is pretty good.