The two-beam coupling method has been a useful procedure to estimate the screening Debye length in a photorefractive crystal. The position and value of the maximum gain value as a function of the photorefractive spacing grating is used to obtain the Debye length value. This position and not the maximum value of the gain, appears to be evidently independent of the optical activity of the material. However, a formal treatment to this effect is not found in the specialized literature. In this paper, we present such formal treatment.