The small angle approximation of the radiative transfer equation is one of the most efficient methods of solving applied problems of atmospheric and ocean optics. The analytical complexity of the radiative transfer equation (RTE) has allowed formulating only primary, qualitative estimates of the accuracy and applicability domain of the small angle approximation (SAA). Another approach based on the comparison of the SAA calculations of light fields with similar numerical RTE solutions is also possible. However, numerical methods of RTE solution for the cases of media with strong anisotropic scattering are inefficient. The approach proposed in this study can be readily generalized to the cases of arbitrary boundary conditions.