there are 2 ways to deal with this:
1) you drain the tank, and take the sender out, and bend the float arm in the direction you think it needs to go.
you would have to re-install the sender and fill the tank with 5 gallons of gas, and then see if you can get the gauge to read 1/4 tank. if not you have to repeat this over and over again.
2) crack open the dash board, release the speedometer cable and most likely you will need to disconnect the main harness connector from the instrument gauges to be able to turn the panel around, there is a voltage regulator that looks like its connected to the board with 9Volt battery terminals, there should be a pot on it that is adjustable.
now that regulator isn't like a modern regulator, it switches on and off fast to produce the voltage you want, so its switching from 12Volts to 0 volts back and forth very fast at a rate that should produce 5-6Volts that feed the gauges. now you can adjust the pot on the regulator until the fuel gauge reads pretty close to what it should read, but you don't want to get the adjustment to the point where suddenly 9 volts are feeding the other gauges. so you need a slow reading analog multimeter so you can correctly get the voltage the regulator is making. a modern digital volt meter is going to read open circuit and bounce around like crazy, you won't get any voltage readings off it.
obviously this is trial and error again, and you have to balance the adjustment with trying to keep the voltage in tolerance so you don't mess anything else up.
now there are alternatives to the original voltage regulator that are digital in nature and give clean voltage instead of dirty ford OEM power, but its expensive and will lead you to rebuilding other things. you could also replace the original regulator with a new one in case the old one is on its way out. of course the readings will be thrown off again and may need adjustments.
i don't like either solution myself and have put off calibration of my fuel gauge.
somebody told me the fuel gauge itself might be adjustable, but that would require even more dis assembly to get to the gauge itself to find out, then you would have to rig up a way to test the gauge out of the dash.
since your gauge pegs at 11 it may require more then electrical adjustment of the regulator and you may need to bend the float arm anyway.
mine reads half a tank as 1/4 i'm usually 6-7 gallons down from F on a half tank, when it should be reading 10 gallons for half a tank and 5 for 1/4.
