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Sikdar et al. [B. Sikdar et al., 2001] has provided a formula to model the window increase pattern given the mechanics of delayed acknowledgements. In this paper, we show that their sum formula significantly underestimates the iterated sum for rounds greater than seven. The approximation error grows exponentially. We also derive a more accurate sum formula whose approximation error is linearly bounded. We show that with use of the incorrect sum formula, cases arise where it is impossible to time out as the initial loss indication in slow start, which can have significant impact on TCP latency, assuming a correlated loss model. We show that our more accurate sum formula greatly reduces the number of cases showing this behaviour, further improving upon the accuracy of the model.