Your fuel gauge may be accurate and the problem could be that the gas station pump shuts off early. Nissan has issued 2 Service Bulletins relating to the fuel tank not completely filling. They are:
NTB05-048, Fuel Tank Difficult to Fill at Times (2003-2005 350Z)
NTB08-082, Hard to Fill Fuel Tank (2003-2008 350Z)
Each bulletin directs that a different part in the tank be replaced. Bulletins are available at the Nissan Help
The problem could also be with the fuel level sensor. There are 2 sensors, one on each side of the tank. The one on the passenger side is part of the fuel pump assembly.