Abstract: The procedure of how to simulate the Ritchey-Common test to derive the relationships between the figure errors of an optical flat and the wave-front aberrations, and to construct a set of influnce functions by ray-tracing program, is here presented. Figure errors of the flat can be extracted from a set of over defined linear equations with the aid of least squares, using the influnce functions and the measured wave-front aberrations for two different Ritchey angles. A test example on a high quality flat 1 meter in diameter is shown.