Swing speed is only one of the factors in determining shaft stiffness.  The shaft optimizer is the way to go at least to get you into ball park of the shaft flex and recommended brand. 

getting a professional fitting is the best way to select shafts for your irons.  You will learn almost everything about your swing and what combo will work best for you.  many courses offer this and so do most specialty golf stores,with many do it for free if you buy the iron set in that store.  Good luck, hope you can figure it out. 

You have to go through fitting, even if you end up favoring the x100.   There are many available and you can also dial in particular life and loft changes depending upon the combos...for example, you might like the Project X 6.0 or 6.5...or s400...and each flex may lead to slight changes in lie/ofts for optimal launch and shot shape.

That's how we do it with our players!

RC