Wife's BE light comes on as you hit the E line. 10 litres left. Enough to drive from Heathcote Valley industrial area to Rangiora AND BACK!!! Very conservatively though.
Average fuel consumption if you DONT drive like a wanker is 13L/100km around city. Better on open road. Around 10L/100km. So in city, give yourself a 50km travel MAX on fuel light.
And while the crap in the bottom of tank maybe bad, fuel filters are cheap and easy to replace. Filling above half for city driving is wasting fuel through added weight of 30-40L of petrol in tank that doesn't need to be there.
That's why needle drops faster in top half of gauge.