Jacs,
The idea is to have less area in the front opening for air going into the cowl and more area in the rear of the cowl where the air flows out. I believe the ratio is 1 to 3. This keeps the air flowing through the cowl and around the engine for cooling. I actually have 2 cutouts in the dummy radial, which will line up with the 2 heads of my 4 stroke engine.
Mike