An analytical expression is developed for the potential drop across the diffuse layer phi(d) in terms of a cubic polynomial in the corresponding estimate in the Gouy-Chapman approximation, phi(d) (GC). The coefficients of this polynomial are defined in terms of the MSA volume fraction eta and the reciprocal distance parameter Gamma. The resulting expression is shown to describe the Monte Carlo estimates of phi(d) obtained in a primitive level simulation of diffuse layer properties.