We present a direct comparison between calculated and measured ionization in Li and Rb when an initially bound Rydberg electron is stripped from the atom by ramping an electric field [0.415 V/(cm ns)]. We describe the method used to calculate the field dependence of the ionization; the method can be used to evolve the population from field strengths where three or more n manifolds cross to the fields where the electron is stripped from the atom. The essential feature of the method is the repeated application of the Landau-Zener approximation for all of the level crossings as the field is ramped. We also give a description of the dynamics of Li and Rb in the ramping field. We observe that the Li selective field ionization is sensitive to the Stark levels within an n manifold, while for Rb only an n dependence is observed.