Note: Remember that the voltage is read by the 8 bit digital to analog converter, and translated into a value of 0 to 255.
This is a big yay - even though the relationship is non-linear, it's darn close to linear, so you'll be able to model it as a linear function. Be happy about this. Very happy. You don't want to try to model non-linear functions; it's a big pain. If you're done and get bored, you can try to tune your arm's accuracy by thinking about the non-linearity of the potentiometer, but you probably won't be able to improve matters measurably over a good linear approximation.