Thats a question thats always stumbled me.
My old MG had a 50 litre fuel tank, i actually ran out of petrol and the car did not start up again. I only got 46.6litres back in. The gauges are never accurate, 1/2 tank doesnt mean 25 litres are left in. IT IS MY BIGGEST PET HATE AND I HATE IT.
35litres out of a 50 litre tank seems way to excessive though. If you properly ran it down into the red, and filled it till the petrol dripped down the side of the car, id imagine you getting 45+ in.