My first fill came when I was showing no bars and feeling a bit nervous. Like you, I was surprised to find I could still have done another 70 miles after refuelling. The warning light came on with 2 bars showing but I knew from my average consumption that I could safely go on until I did 500 miles. I filled at 500.8 miles. So it looks like 2 bars is an early warning and zero means do something about it now! The manual cautions quite strongly against running too low to avoid "damaging the catalytic converter"