Most sat-navs will provide the PS locations - thats what I use and try not to let the tank go below the fourth 'dot' on the 370 guage.
That said, when I had the GT Edition the other weekend the range told me I had run out on the M25, but 20 miles later it was still running before a fill-up. Its reckoned you have 50 or so miles left after the the range says nothing - dependent on how heavy your right foot is of course and I have been known (but rarely) to feather it