The 3x exit air is just a general rule of thumb.

What you should be more concerned with is making sure the exit hole has a little lip or air dam to disrupt the air passing over the outside of the cowl.

By disturbing the air as it passes over the bottom part of the cowl where the air is SUPPOSED to exit - you can create a low pressure air dam to help SUCK the air out of the cowl. Air entering the cowl doesn't mean it's necessarily cooling the engine. Air can go into the cowl and become pressurized. It can become stagnant.

The first picture is a rough idea of how air can enter the cowl and become stagnant. The air slipping past the bottom of the cowl does nothing to help EXTRACT the hot air from inside the cowl.

The 2nd pic shows how a small lip or air dam can help create turbulence or low pressure right near the cowl exit. This helps pull air out of the cowl and improves flow of air through the cowl.

Air don't do ya any good if it ain't moving through the cowl.

So do you really need 3x the exit air? Maybe. But you'd be better off, regardless of the exit opening, if you put a small air dam on the cowl to create that turbulence and low pressure spot.


EDIT:
I'm not saying a cowl without an air dam doesn't work. You'll get SOME cooling, regardless.

I'm saying an air dam HELPS to extract the air. It helps keep air moving through the cowl. Cool air in. Hot air out.