on my "R" the fuel light comes on and off for about 10-20 miles - usually somewhere between 130 and 150 miles and but then stays on. The fuel countdown seems to be clever enough to take account of each period of "on" as it doesn't reset to zero until I fill up.

If I catch it as the light is staying on a refill takes 11 litres meaning there is around 4 litres in reserve, about 50-60miles.