If you slide the float to the top does the gauge read half full or full? Is the float sticking halfway?
Sometimes the twisted metal part of the metal slide mechanism can have a spot that the float will stick. This is usually because there is a bit too much twist in one zone for the float to slide freely. I carefully "un-twisted" the metal in the area that it was sticking and it never stuck again.
If the calibration is off then you may be able to adjust it. Someone else here may know how to adjust the calibration or if it is possible.