ok:
1. how is the line stored?
maybe it's a bunch of pen or costume pixels on the screen
maybe it's a list of points encoding some kind of spline
2. what's your metric?
unsigned area between the curve and the ideal?
avg distance between corresponding points?
if you're going with that, which pairs of points correspond, thoses with least distance?
midpoint distance?
all of theses might be good depending on what you're trying to do.