As for what data do you look at for fine tuning a shaft? For me its trial and error. Not really a set data point. What is actually working for that golfer. What are they trying to correct. What does the ball flight look like. How is the spin responding to flex change.
From my own observations the more seasoned players are the ones who notice slight changes in flex that can be brought upon by hard stepping or soft stepping. They have been playing long enough to know how they like a shaft to feel.
Certain shafts like the new Xcaliber Rapid Tapers i have been working with have a very defined ‘Kick’ in the shaft. Their unique geometry has a distinct way in how it plays. Fine tuning this shaft by hard or soft stepping will either deaden or wake up that ‘Kick’ in the shaft. So fine tuning this particular shaft pays off big dividends.. Some other shafts are far less responsive. With a lot of steel shafts I see players very happy with factory A, R or S. With time a player adapts his swing to his combination. As with any setup there is a learning curve. The hard or soft stepping is a tool to get that combination as good as possible.
Golfers just starting out simply just seem to respond to a visual result. Inconsistent contact has feel all over the map. A Seasoned golfer tends to prefer feel first and then results as they can adapt the swing. I wouldn’t say fine tuning a beginners shaft is not valuable. But with an unstable and developing swing they will never know the difference between an R flex and a hard stepped R
Again this is just my experience, every golfer is different when fitting. The more you work with a particular shaft the more you understand it and can pass that on to customers or yourself. Shaft experimenting can certainly be fun but it can be brutal on the swing and sometimes you get lost with all the data and choices. No shaft will perfectly fit a golfer. And there is no miracle shaft. But getting that shaft to feel and perform well enough to give the golfer confidence is key.
If I know a shaft well enough and I have a golfer with a certain swing speed and tempo I know what will work for him at a given flex. I will make up a club that should work for his swing. With observations and feed back ill build that set off the one test club.