If there were any standards defining shaft flex, this would be an easy answer. The problem is that a stiff in one model can be stiffer than an X-stiff in another or softer than an Regular flex in yet another. For better or worse (probably worse) flex is really just a catch-all that loosely (or barely) describe the overall stiffness of the shaft. As a single measurement/metric, it’s not particularly helpful as far as describing where a shaft is stiff and where it might be a tad softer. Some shafts have stiff butt sections and soft tips, some are soft in the handle and mid sections and stiffer in the tip. Shafts come in all combinations and again, a simple flex designation doesn’t begin to cover it. All of that said, if the stiff in whatever shaft you’re playing is working better than the X, that’s probably where you need to be. Swing speed is just one factor in shaft fitting. Tempo and transition are important as well … and that’s before we dig into the weeds on swing path and miss tendencies. I’m aware of at least one study that found that the shaft that feels best in a golfer's hands is often the one that performs best as well. If it feels right, it’s probably because it’s bending right. So, while we’re always going to side with the data, there may be something to the feel thing when it comes to choosing a shaft.