Also there is an airspace always left at the top of the tank, note that the filler neck is not at the tank's highest point. Failure to leave an airspace there could cause nasty tank pressuring issues if you put in cold fuel on a hot day.
For the low fuel light, that's a function of the sender. If you look at the sender, there's a separate one which looks like a little microphone on a stalk. That one is for the low fuel. It is (From what I deduce) a thermistor, so it's performance will vary from car to car and by ambient temperature. There is a constant current running through this from the gauge fuse, so I surmise that it remains cool while immersed in fuel, and warms when it's exposed to air. Getting splashed with fuel is probably adequate to cool it as well, so it will take longer to come on if you're driving on rough roads than smooth ones. (How many of you notice that the low fuel light tends to come on when you come to a stop to park?) It's therefore going to depend on a number of factors as to when it comes on and stays that way.
