The pot is adding resistance throughout the range of the rheostat on the sender.
It will work in that one spot. The problem with the added resistance is that it becomes a set value. The fuel sender acts as a varible resistor changing value as it moves. So you are adding the same set value to each on the diminsing values. That's why matching sending units to the guages are critical.
But in any case getting back to the original problem, his sending unit is offering too much resistance, which when full it only reads 1/3 tank. Adding more resistance is only going to lower the reading, not raise it.
