Keep the strakes parallel all the way. If you were doing full cf mold you might benefit from molding them together to a point in the center, but unlikely. The point is to have the flow as smooth and linear as possible while accelerating the airflow under and out of the car.
Unless you have one crazy massive wing mount hanging it back several feet from the rear of the car, you're not getting any activation of the wing from the diffuser, so it's all about drag and air speed underneath the car. I'd put some small strakes at the end of the blades or just to the inside of the tires as well. Keep the air underneath from exiting along the doors and wheels.
As I said on Facebook. In all my models I lost attachment after 12 degrees or so, but your mileage may vary with real world testing. You can always use the string and GoPro test method
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