Some of us need to look over the top of our glasses when our head is down low. The limiting factor is the brow of the eye port on the helmet.
Years ago I was told to make sure the front fairing gave me frontal coverage when I was on the bike in a position I could actually ride in, rather than one I hope to assume. Experience on rough tracks showed me that I have a hard time when my chest is on the tank. I get bounced around a lot and it is hard to keep balanced to ride in a straight line. The best position for me is with my belly resting on the back of the tank and my chest slightly above it. My head stays relatively stable so I can concentrate on keeping the bike on line.
Rose took pictures of me from the front when I was on the bike. Using those, I sized the fairing so it gave me coverage. My hands, shoulders, etc are all behind it. Only the top of my helmet sticks out above the top of the windshield. The trailing edges of the windshield and fairing were made as close as possible to being parallel to the long axis of the bike where the are broke off of their trailing edges. Now I had a fairing to give me coverage. Fairing surfaces were carefully looked at to minimize those that would give me lift and to maximize the those that would push the front end down.
Not done yet. It was time for fine tuning the front shape and incorporating the tail. This is where the NACA profiles were looked at. The top and sides of the bike have different profiles. The fairing shape was fine tuned to make the best use of a NACA profile. The tail was done last and it followed a NACA shape as far back as it legally could. This is 1/2 the rear wheel rim diameter past the back of the rear tire, for FIM. The front fender was also looked at and shaped to blend in with the front of the fairing.
The front of my fairing looks too be too big. Note on today's post about Erin Hunter the size of the fairing on Andy Sill's bike. Current thinking is to have a good shape and coverage as opposed to a smaller shape with less than optimal coverage.
This might not be the best way to do it, but it worked for me.