I have always used the rifle stiffness ratings as a guideline when building clubs. Where would a standard rifle 5.0 appear on your graph for iron frequencies without a grip? I built a set of irons using 5.5 tour flighted rifles(not project X) and Golfworks cavity backed forged irons 12 years ago. Those shafts are amazing to put it mildly but they are too stiff for me now. If I could find a set of 4.5 tour flighted rifles I would buy them in a heartbeat.