Non Rallye fuel gauge either full or empty
I would start by roughly plotting resistance through the movement, what you are looking for is a sudden increase which indicates damage to the resistor or corrosion/ loose/ poor connection between the wiper and resistor
Maybe I'm misunderstanding.........are you saying you have the gauge properly powered up in the cluster, and have CHECKED the sender resistance, and the gauge still not accurate?
What resistance(s) are you using?