Turn off ignition and then remove the instrument fuse. This may reset the fuel level calculation function. It might take this and another tank top up to reset this.

The tank on the 40 series has some oddities in its shape that make the fuel level sender not capable of measuring all of the fuel. The inst cluster calculates fuel depletion after sender reaches bottom and fuel depletion from an assumed complete top up if sender goes to full top range after having been lower on last key on cycle. This depletion calculation is based on the fuel consumption signal sent by fuel injection and on distance traveled from speedo signal.