Proof stress is a property of the material, you compare your calculated stresses for the component against the proof stress to see what safety margin you have.
probably one for the eng'g s/sheet forum, but you'd think it could ... it's easy to determine the elastic modulus, thus it's easy to define the 0.2% permanent set line, so "all" you're looking for is the intersection of this line and the stress/strain curve. i'm sure there are better ways but a simple way would be to calculate the 0.2% permanent set line at each point you have for the material stress/strain curve, like stress = 30ksi, material strain = 0.003, permanent set line strain = 0.005; and later stress = 70 ksi, material strain = 0.08, permanent set strain = 0.009, and interpolate for the cross-over
