My tank is doing this, too. Shows about 3/8 of a tank when completely full, then drops to 1/4, then slowly goes down to just below E when it's empty.
I just put a label with the letter F over the 3/8 area, and put black tape over the 1/2 and F.
Unless this tank starts leaking I'm not going to bother to pull it to check this out. If I *DO* get a leaky tank, it's getting a new tank, new sending unit, new sock, new lines, everything...*THEN* it should work properly.