The fuel level sensor in the tank is not a high accuracy device. If the sensor gives inaccurate readings, then it does not need to be adjusted, but simply completely replaced.
Removing the fuel level sensor
1. Remove the tank as already described.
2. Unscrew the safety ring (note: left hand thread!) using a gas wrench or two wide screwdrivers inserted into the grooves of the safety ring on the top of the fuel level sensor. Do not damage the safety ring while doing this.
3. Install a new sensor seal as it will be damaged when the sensor is removed.
4. Before installing the sensor, clean all sealing surfaces.