"Caltech Calibration Toolbox" by Jean-Yves Bouguet is one of the most famous open-source calibration tools [1]. Although it works well for general camera lenses, it reveals problems in case of wide-angle lens. This paper analyzes the problem and shows that the insufficient calibration of radial distortion is caused by lease-square based polynomial fitting. To overcome the problem, this paper proposes novel refinement method using inverse mapping-based extrapolation. The proposed method is expected to be an efficient supplement and is opened to the public in the appendix as a short Matlab script.