My suggestion: Abandon all your measurement strategies.
Because, every rider is unique, as to height, riding position, physical proportions, and preferences. Each windscreen has its own definite characteristics. The only way I know to evaluate and reconcile a given rider and a given windshield's performance characteristics is: Try it out!
Some may like a particular shield; another may experience intolerable buffeting from the same product.
I'm 5'-11: My TPI screen suits me fine, but . . . the 6'-2" guy I bought if from couldn't STAND it; buffeted him right in the face.
So, try before you buy, if you can--if not, be prepared to re-sell and try again!