I don't know where the theory came from that you have to see over the windshield.  I am 5'10" and use 2 windshields.  I have a 12" memphis shades for long trips.  I am looking through it about 3" from the top.  In rain at 60mph, nothing catches me in the face or head.  I use "rainex" on it, so the rain just rolls off, and I don't have to fight wiping the water off my glasses.  At 85-95 mph, it creates a nice vortex of air keeping me cool, but not whipping the wind in my face.
For normal riding, local day trips, and back and forth to work, I use the memphis shade 8".  It looks cool, (smoke, with mirrored lens) and it is functional.  I have used taller windshields for years, and have always faired better than riding partners in the rain.
A point in fact, Tuesday, I had to ride back from one of my offices during a severe thunderstorm.  Wind gusts to 60 mph, and some of the areas of rain I rode through was very heavy.  It was a 90 mile ride, and with my rain gear, no helmet, skull cap, bandana over my face, I really only got wet on my feet. (No lowers on)
This is a matter of personal preference, but I have made it through some tough weather with this type of setup.