Abstract: Jacocks' method has been improved by deriving a difference correction matrix of second derivatives for Gauss-Newton iteration method. The numerical tests justify that the improved method enables the calculation to converge more rapidly and reduces the CPU time. The combination of the distortion prediction described in reference[4] with Jacocks' method avoids the necessity of experimental extreme values as input data for the Jacocks' method. The comparison of the distortion prediction results with the experimental data demonstrates better agreement.