Ride along at a decent speed and hold your hand up above your head and find the air stream. You can then get an idea of how high you need the shield to get the air up and over you. One nice thing about the Madstad is you can run it a little lower than normal and tilt it forward and that helps push the air up higher. I run the 15 inch front all the way down and slightly forward and it gets me clean air for what ever helmet I use and i'm 6'4" tall.