A few thing to note about shaft flex. There is no standard in what makes a shaft stiff vs. regular. One brand's stiff can (and in some cases is) softer than another brands R. Also some companies now make multiple levels of flex. Ping makes soft R, R, S, tour S, tour XS.
Also, swing speed is only one indication of which flex you will need. If you have an aggressive transition from the top of your back swing down to the ball, you will load up the shaft more so you will need a stiffer shaft to decrease shot dispersion. However, I have met people who use "R" shafts to hit the ball way past what I can hit it. I played a few weeks ago with a college kid who hit a Callaway Octane driver in 10.5 /w R flex. He hit it to the moon in terms of height, but got a TON of carry and very little roll. I asked him if he ever thought about trying a 9.5 in S to see if it can bring the height down and maybe give him some roll out distance. His response was, why would I ever need to hit it further?
My point is simply this. The letter written on the shaft means alot less than people think it does.