I'm pretty confident that a linear approximation should do nicely... in terms of real angles (not pot values), you're trying to make it so that theta1+theta2 is always constant. How does this look?
Code:
int wristPos(int elbow)
{
return elbow*(350-900)/(600-350)+1670;
}
Calculate the slope of the line, multiply it by x, add the intercept, and that gives you y.