I would've thought that l/100km (on the tank refill average number) would be also an indication of you driving style? Not sure if there's a formula for calculating the range from which ever number but doesnt look like its the tank average l/100km.
From experience it might be calculated based on maybe the last 30min of l/100km rather than tank average.
I got to work yesterday with 18km left on my range (work to home is about 15km). So I thought I would make it home without the range reducing to 0. After getting in the car after work and starting it up, the petrol engine started (as usual on a cold start) and the l/100km reading shot up to 30 for the first few minutes. All of a sudden, my range dropped from 18km to 0 and did not bounce back up. I ended up driving home with a range of 0 all the way but I did make it home. I decided to not go any further and filled up straight away when I got home (filled up 36L). Odometer reading was at 733km which wasnt even my best mileage.
So ..... conclusion, I do think that even after your range hits 0, you prob still have about 15% of fuel left which at my consumption rate would still take me 100-200km - in this case I still had 9L left in the tank which should take me 150km.