Its probably best to just fill up and not find out since it'll most likely result in a buggered fuel pump which are pains in the arse to replace being in the tank. Thats when you run out of fuel and look like a fool :whistling
fuel light to you're walking is either 6l. personally i try to never let it get to the light because a)my fuel pump howls when level is low and b) the fuel pump is cooled by its immersion in fuel. no fuel= no cool= pump is for decoration only