The BB6 fuel tank is 60L.
The fuel light comes on when 45L has been used.
At this point you have 15L left in your car.

Are you doing ACTUAL fuel consumption figures?

Fill tank
Reset trip meter
When empty fill tank again
Record K's traveled
Record fuel put in

Then the math is K's / Fuel

So you put 45 Liters in and you've traveled 400K's

400 / 45 = 8.8

So 8.8 K's per liter of fuel.

My apologies if you already know how to do this but so many people just have no clue.