I found this in the site above, and its quite interesting. You might find it useful:

"What the Manufacturers Don?t Tell You ? Stiffness Rating
Manufacturers will tell you the diameter and pound test and maybe even the color, but they don't tell you a stiffness rating.? Mason mono is hard, we all know, and Orvis SuperStrong is soft, but you couldn't tell by the names or the packages.

A simple method to determine if the stiffness of the leader material matches the stiffness of your fly line is to bend a section of each in half with your fingers and "feel" the approximate stiffness of each. You?ll easily be able to feel the difference in the resistance to the bend. If you use a butt material that is too flimsy, you?ll experience the "hinge" effect when you cast. The leader will not turn over properly and hinge where the fly line and leader connection is made. Get over this hurdle and you?re well on your way to a designing proper leader."