Yeah it's really not that precise, it's just a part mounted in the tank that keeps the light off when it is covered in gas, when it gets uncovered the light comes on. The car must be perfectly level for it to correspond to a known amount left in the tank.

Besides it doesn't matter how much gas is left in the tank when you decide to fill it up, you never were going to use that gas anyway. It's dead weight.

The important part is how much it takes to refill it, and you get that from the gas pump. You need to keep records of overall how many miles you drive and how many gallons you put in. Every time you return to the same status of the tank, be it empty or full as long as it is the same amount as when youlast wrote down the mileage, you can calculate the exact mpg averaged over the last tank or more.

 

If I am driving on the highway in my 2000 Ex or 2000 PreludeSH I know I can go at least 40 miles when the light comes on. When driving in the city, I always fill it up when the light comes on.