The paper proposes a method for calculating the rheological parameters included in the standard power ratio of superplasticity based on the results of test forming of hemispheres using a cylindrical matrix. The method takes into account the value of the entry radius of the matrix. The analytical model of the process is developed based on the main assumptions of the theory of thin shells, the finite element model is executed using the ANSYS10 ED software package. As compared to the traditional approach, the proposed method allows improving the modeling accuracy with an error less than 1%, not only for the forming time, but also for the thickness at the pole of the dome.