I tried all the Math node settings and I liked how multiply worked the best. I had a smoother transition from clear and shiny to rough by tenths for 0.100 to 1.000. It seemed like a very smooth change from one to the next without the big fall off from 0.10 to 0.20. Power worked too but it seemed do go rough and diffuse quicker than the multiply did. I set my multiply value to 0.200 and it seemed to work the best. 0.100 did not quite get to rough by the time the Value was set to 1.000.

So, here is my node tree:

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I found a similar situation-the best I could manage was to get power work the same way as multiply.

May be a difference in Blender 2.79?