This subject is really hard to solve without a full scale wind tunnel. Too much angle the airflow going out of the diffuser it disrupts the spoiler but you can get away with the angle by extending the mouth out away from the rear of the bumper. The greater the angle increases the total volume of diffuser. You might not need that much because the spoiler works with the diffuser. Too much of one makes the other less efficient, balance is the key. I would make the diffuser mounts adjustable so you can tune it rather than making it with one specific angle.
Here is a good place to read about different designs and the physics behind it all.
Diffusers | Engineering basics | Aerodynamics - Racecar Engineering