The exact expression of F+ is computable through many different methods [1-4] but is too much complicated to derive explicitly ∇F+. To overcome this problem, we propose to consider an approximation of F+. As said in introduction, many kind of approximations are available (Gaussian, binomial, compound Poisson or large deviations). In this paper, we have chosen to use a binomial approximation as it provides an expression which is analytically differentiable and is known to be a good heuristic to the problem [8].