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We describe work on the development of an optical link budget tool for an intensity-modulated direct-detected photon-counting channel utilizing pulse-position-modulation (PPM). We provide new material in several areas. First, we provide an approximation to the channel capacity, which is not known in closed form, to enable efficient search algorithms over the trade space. The expression also illustrates clearly the trade-offs between signal and noise power, and modulation parameters. We provide an approximation to the losses due to log-normal fading, which may be used to model scintillation. We provide approximations for the loss due to photo-detector blocking and jitter. Lastly, we describe a methodology to choose an optimum detector sub-array in the presence of dark noise, blocking, and an arbitrary point spread function.