Based on the Oseen and Frank elastic continuum theory of nematic liquid crystals, and on a generalized torque balance equation at the interface, the magnitude and the gradient of the tilt angle of liquid crystal molecules at the interface have been calculated in a homogeneous cell as a function of applied voltage and liquid crystal-to-wall anchoring anisotropy C. Consequently, the reflectivities associated with the excitation of the surface-plasmon polariton propagating along the liquid crystal-to-wall interface can be evaluated for comparison with experimental results to determine a value for C. Numerical computations of reflectivities have been shown to fit the experimental results of Sprokel, Santo, and Swalen [Mol. Cryst. Liq. Cryst. 68, 29 (1981)]. An average value of 0.11±0.05 ergs/cm2 for C has been deduced.