Numbers sound right
If you look at your gauge, it will go past the "F" mark, "F" is only 80% of the tank capacity at a certain temperature. So "Full" is 80% of 18 gal or 14.4 gal.
But half full will still be half of the total tank capacity or 9 gal.
So when you 'filled' a half tank with 4.3 gal..... 9 + 4.3 = 13.3 I think that is pretty close to the full mark of 14.4 gal
And as was mentioned, the indicator lights are really crude....
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