I talked with Jim, our resident super expert on such things, and he said this:
“With steel shafts I always say soft stepping one shaft is in the 3 to 5 cpm range depending on which steel shafts. True Temper and Nippon will be more like 15 cpm’s between flexes. FM Precision always said ½” was equal to 3 to 4 cpm’s. With graphite shafts 10 to 12 cpm’s between flexes for most shafts so soft stepping one club could change from ¼ to ¾ of a flex depending on the shaft.”
My experience in doing this myself would concur pretty much with what Jim said. Maybe a 1/3 to 1/2 a flex in steel, depending on the steel and 1/4 to 3/4 depending on the graphite. All of this assumes the proper head weights and head weight progressions, and consistent hosel bore depths.