First, without an original style 16 gallon tank I doubt you will ever have an accurate fuel gauge reading.
Second, with the correct tank the big problem with a fuel gauge is how the arm on the sender is bent. If it doesn't move the potentiometer over the proper range of resistance (~73 - 10 Ohms) then the gauge can't possibly display correctly.
Third, it is possible to get a sender whose resistance vs position is a linear function. Ford used a very NON-linear winding on the sender to match the non-linear response of their gauge heads.
If you get through all those problems then the adjustments should be to make the 0 and 100% needle locations (E and F) correspond to the max (no gas) and min (tank full) resistance values that the tank sender will produce.
Reading all that it should not be hard to understand why many gauges are inaccurate. My original sender was pretty accurate, considering, but it quit working. I will try all the above for the new sender I plan to order. We shall see how it all turns out.
John,
I would imagine that the zero should be adjusted first and then the span. Is that what you have found to work best? | 
