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Downlink coverage issues for macrocellular users (UEs) in a macrocellular network with Closed Subscriber Group (CSG) femtocellular overlay have so far been investigated almost exclusively via simulation. However, consideration of specific scenarios via simulation may not provide general insights applicable to long-term planning for femto-macro deployments. In this paper, we employ general models for macro Node B and femto access point (AP) locations to derive analytical expressions for the probability that an arbitrary UE cannot be served a macro Node B because of interference on the downlink from a nearby femto AP under CSG, and for the probability that the UE is served by the femto AP instead, if the femto AP operates with Open Access (OA). These expressions depend only on the ranges and densities of the macro Node Bs and femto APs, and on the parameters of the wireless channel model. We then show how to apply these results to compute measures of the fraction of overall macrocellular traffic that could be offloaded to an OA femtocellular network, thereby reducing the load on the macrocellular backhaul and increasing overall system capacity. These results are of interest to operators weighing the capacity vs. coverage tradeoff of a femtocellular overlay on a macrocellular network.