Why 0.2 percent is used to find offset yield strength?

Due to the difficulty of locating the precise point at which the slope of the modulus line begins to change, indicating the onset of plastic deformation and the yield point. By placing a line parallel to the modulus line it is much easier to see the precise point at which they intersect. The selection of .2% is just a convenient distance to provide a good visible intersection and assure a very reasonable degree of accuracy.