[JesseK]

Even if the potentiometer reading isn't linear (i.e. there is a linkage that connects the potentiometer to a point on the arm), it's fairly simple to get a good equation. Plot several calibration points in Excel, then use the "Add Regression Line..." option by right clicking on the graph. Then select the various equation types until you have a line that best-fits the data. Selecting the 'Show Equation on Chart' will give you the equation to program.

Just make sure the potentiometer readings are on the X axis.