I made a template out of a piece of thin cardboard. Draw a curve for one side (center out), then flip it over to duplicate the curve on the opposite side. I started with a shallow cut, and just went gradually deeper until I got clear air at my helmet. A taller rider would want a shallower cut, a shorter rider a deeper cut. I would recommend doing it this way to get it right for you. For me, this design eliminated the turbulence, except in certain side wind or very heavy headwind situations, while still providing some protection to torso and shoulders.
I made the cut with a dremel tool using a "zip" bit, which looks like a drill bit, but is designed to cut sideways. They're available at most hardware stores. Smooth the final edge with a flap sander on the dremel, and use car door moulding to finish (available at most auto parts stores).
