Space-based coronagraphs for earth-like planet detection will require focal plane wavefront control to achieve the necessary contrast levels. These correction algorithms are iterative and require an estimate of the electric field at the science camera. In order to maximize science time the correction time must be minimized, which means reducing the number of exposures required for correction. This also means minimizing the number of iterations and the number of exposures per iteration to achieve a targeted contrast. The ideal choice is to use fewer exposures to estimate the electric field with the same level of accuracy. We demonstrate an optimal estimator that uses prior knowledge to create the estimate of the electric field. The performance of this method is compared to a pairwise estimator which is designed to give the least-squares minimal error. This allows us to evaluate the number of images necessary to achieve a contrast target.