Man, that's the 64k$ question ! Most of these guys have 4 or 5 windshields and are still lookn'. I have 4 for my Roadglide myself but here's some suggestions anyway.
Get the right heigth, your riding line of sight should be 2" higher than the windshield.
Get the right configuration, If ya ride solo ya don't haveta' worry about protecting the passenger.
Are ya sure you've change everything your gonna? If ya put a new seat or handlebars on.....you'll sit different.
If ya can......try em out, don't be shy about ridin yer buddys' bike a coupla blocks to see if it REALLY does do what he said it does.
Shop around, Windvest (Many sizes, 3 shades, recurve top, better passenger protection)
Clearview, (Not many sizes, limited tints, recurve top, thick, very scratch resistant)
Klocwerks, (Not many sizes, limited tints, recurve top, stylized, poor passenger protection)
Harley's got a ga-billion of em but no recurve.
And, remember the re-curve one's "ride" anywhere from 2-4" higher than they are so if your line of sight is normally 8" over the top of the fairing you could get away with a 4" recurve.
Just my suggestions, for what it's worth.
Good luck!