Fuel Guage Calibration

I know there has been discussion before, but can someone tell me how
to correct the fuel guage on my Sprite.
It reads full, with a full tank, and stays that way until nearly
empty.
It then wakes up, thinks "Oh S**t, if I don't do something I'm going
to running on dry" and the needle begins to fall, rapidly, towards
zero.
I get about 20 miles before the engine starts to misfire.

Guy