We consider the hypernetted chain approximation to the interaction free energy between charged walls in a monovalent electrolyte, emphasizing the practical advantage of using the mean spherical approximation for the bulk ion correlation functions (the MSA theory). The decay rate of the interaction potential is largely determined by the bulk ion closure assumed, and in comparison with other closures, the asymptotic decay of the MSA theory is more rapid than given by the classical Debye length, agreeing well with the decay in the hypernetted chain (HNC) and the improved HNC (including bridge functions) models and with exact calculations. Common to both the MSA and HNC theories, the magnitude of the asymptotic interaction potential is underestimated, due to the inaccuracy in the hypernetted chain wall-ion closure approximation. Because of the semianalytical nature of the MSA closure, the MSA is numerically more appealing for practical implementation in an analysis of experimental data. For such purposes the Poisson-Boltzmann theory and the MSA theory provide lower and upper estimates, respectively, of the surface charge. By using both in comparison, one obtains a far better appreciation of the true value of the surface charge operating in real circumstances.