Not very scientific but I just made it as big as I could while still keeping the transition from the front of the car smooth (still not as big as Porsche 911 RSR's hood extractor (see below)). Also nothing scientific about the radius, I just cut and bent the hood to where I wanted it (again smooth transition from the opening in the grill) and then cut a punch of cardboard up to make templates. Once I had templates I liked I traced them onto aluminum and used a jigsaw to cut the shapes out. To make the 90s I used a dremel to edge in a line and then used a hammer the 90 degree bends in. If you moved the brake fluid reservoir you could get closet to the Porsche design
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