Fuel gauges aren't known for their accuracy. The marks on the gauge are only a guide - in particular, F doesn't necessarily mean Full, and E doesn't mean empty.
It's usually sufficient to have a good idea what's happening when the gauge (or warning light) indicates that you're about to run out; many drivers fill up when it shows ¼ full, or when the warning light comes on. Personally, I keep an eye on the odometer, as I know the range either in the city or on the highway. Oh yes, there's always the trip computer with DTE (you need to confirm this for yourself.)
BTW, I've found that the Cerato fuel gauge is quite accurate; I can estimate to within 2 litres from what the gauge says. This is the exception.