If we assume that these 75% of patients had a mortality rate that was the average of the early start and late start groups (i.e., 31%), then how much power did the study actually have under the same assumptions as in Problem 10.88? 

One issue is that not all the late start patients actually started dialysis when their GFR was between 5.0 and 7.0. Indeed, 75% of the late start patients initiated dialysis at a GFR > 7.0 due to symptoms.