Cowled Engine Cooling?
As mentioned, 2x outlet area as inlet area. However, if you tend to put the largest size engine recommended like I normally do, cut a little more out. I had a problem with my World Models Midget Mustang with a OS 91FX leaning out in flight. I would take my cowling off and it would run fine. I decided to cut more of the cowling out and allow air to move throughout the cowling and especially around the engine. It runs fine now.
A buddy of mine made the mistake of not providing enough ventilation with his EZ P-51 with ST 51. He tried to keep it as "pretty" as he could and when it leaned out at low altitude in a bad spot, it went spiraling to the ground.