The usual ratio for hot air exit for cool air inlet was previously noted at 3-1, meaning triple the exit area for hot air compared to what is provided for cool inlet air. That ratio is adjustable between 2 and 4 to 1, but that adjustment is conditional. The lower ratio can only be used if you have done something to accellerate the exit air flow, such as the construction of a ramp at the beginning of the exit area outside the cowl.

For an example of how air moves, go out to the garden and lay a water hose on the ground. Turn on the water and watch the way the water moves over and around obstructions. Place a few small rocks and other items in the way of the flow and observe how the water adjusts to flow as freely as possible around and over the obstructions. The water always prefers to go around an obstruction, and that water can be directed to flow where you desire it to with the additition of dams and barriers for deflection. Plan your cooling air system with those observations in mind. You will have used a short course in fluid dynamics to have built your cooling air system. A rather good one since it was based on visualizations and practical experience.