If the gauge feeds off 12V direct, and uses a negative to sense fuel from the sender inside the fuel tank - get it fixed from a proper gauge mechanic, be sure to take along your gauge face too, that way he knows what resistance shows what. and full actually shows full, - the rest usually is calibrated itself
If the gauge is going flaky itself then some work can also be done inside the gauge, usually the bobbin inside becomes crudded and shows problems. A dip in something like WD40 and circuit degreaser works - be sure to coat it with a small spray of PCB varnish.
My 1302 had this problem that was fixed by cleaning out the sender in the tank. My VW Golf had this problem with both heat and fuel gauges which was sorted by replacing the voltage regulator behind the gauge cluster