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In a recent study, the error performance of pulse-position modulation (PPM) schemes has been investigated for a terrestrial free-space optical (FSO) link, which was assumed to be impaired by both turbulent atmosphere and pointing errors. Only for the binary case a closed-form solution was achievable, whereas for non-binary conditions just an approximate relationship could be given. The key point in this respect was the approximation of the complementary error function so that the two-fold integral, otherwise required in this context, ended up in a closed form. In the current contribution, the problem is tackled by an additive combination of exponentials whose coefficients are optimised in the least-square sense. It turns out that second-order approximations are a good compromise between accuracy and computational complexity.