Don't know what you're trying to get at, but pressure and flow are interactive. There must be a pressure differential between the inlet and outlet, or else there will be no flow.

Creating an oversized outlet in a model engine cowl does help minimize backpressure, which is why you often hear the "twice the inlet size" recommendation. If one does not possess more in depth understanding of fluid mechanics, such rule of thumb is a fairly safe bet to go by. However, as we should all know by now, there is always a drawback. An overly large outlet will result in more drag (as I explained previously). Drag is a major concern for full-scale aviation. For most RC applications, however, drag is not a big deal. Heck, when you make the outlet as large as possible - that is, the size of the cowl - the airplane model will still fly just fine. How big to make the cutout depends on how technical you choose to be and what you consider esthetically acceptable - the latter of which is a non-technical decision.

Let me make it absolutely clear that I am in no way advocating doing to our RC models what Mark Langsford has done. He has his full-scale design concerns, we have ours. The two sets are not always the same. The engineering principles at work, nevertheless, apply universally.

I bought up Mark's example only to counter the incorrect assertion that NACA duct only applies to fluid inlet on flat panels.

NOW, For model considerations, the size of the cowl oulet is but one consideration. Equally important are the location and configuration of it. I have seen people hog out the bottom of a cowl, much of which faced forward. In general, it is far better to have the cowl outlet face rearward, such as with a oversized cowl that extends below the fuselage or a "tunnel" that recesses into the fuselage inside of the cowl.