According to most wind screen manufacturers, you should just be looking over the shield (unless you WANT to look through a giant one). The best way to measure for optimum is to sit on the bike and lay a yardstick over the windshield. Have a friend stand back to "eye level" the yardstick. It should hit you between your mouth and nose.
Staffo, I'm 6' and have the same Clearview Summer Screen replacement which is just tall enough for me (that's my bike in the pic you posted). They now make a 18" which should be perfect for you. You probably wouldn't have any trouble selling your Clearview to someone that has the Triumph mounts. It is one of the very best screens made for our bikes.
