I suppose that if you are using your fan connected directly to a 12V power supply you are using 100% duty ratio, which would be maximum capacity.
The louvre I have is very light, and if you blow air through it, it easily opens. I think 74 CFM would be enough for that.
It is good to use it so that you limit the air exchanges inside the box (better control), and also prevent insects from coming in. But it's not extremely necessary.