I don't know if this helps but I found working the other way is easier. I set my water to a mirror image first then slowly adjust things until I get the scene I want. To get a perfect mirror, drop all values on the waves tab in the Water shader down to zero.
Set index of refraction to about 2.4 I find gives me a nice mirror, you can go even higher if you want but then the water will start looking like a metal surface.
Hope this helps!
JR