Hi

I ran into the same thing last night. The light came on, I filled it up and I only had to put in 38 L. (1 gallon=3.8 Liters)

The tank is 50 Liters, so 12L left.

My Civic used to have 8 liters left when it first showed empty.

It is always good for the car to refill it after it hits 1/2 a tank.

Take it easy