A performance metric is described that can be used to determine the parameters of a partially coherent beam for near-optimal performance of a free-space optical link. The metric is defined as the mean received irradiance minus the standard deviation of the irradiance, and maximizing balances the effects of beam spread and scintillation. An analytic form of is developed for a Gaussian Schell-model beam, where the optimization parameter is the transverse coherence length. Comparisons of the metric performance are made with the more conventional probability-of-fade metric. The metric is applied to determine the characteristics of the optimized coherence length as a function of a variety of link parameters and scenarios. In general, the optimized coherence length tends to decrease with increasing turbulence strength and propagation distance but increases with wavelength, although the behavior of specific scenarios can vary.