Unfortunately the 3 to 1 rule is not a good starting point. Pe Reivers had a really, really good forum that had research, examples, and information regarding cooling of model engines (as well as dis-proving the 3 to 1 rule). I just noticed that it has been removed, so unfortunately I can't hyperlink to it. Sometimes it works out that the 3 to 1 rule will work in a particular installation, but it has nothing to do with the rule itself being correct. It is always best to use telemetry or testing when you begin cowling an engine to determine if you have enough cooling air moving through a cowl. Pressure differences cause air to move from one location to another. One good way to create a low-pressure zone is to add a ramp (or louvers) to an exit hole. This does not always make sense for a scale airplane, but it is an effective way to provide some airflow.

Keeping track of temperatures when cutting holes in your cowl is the best way to monitor the effectiveness of your cooling. There is no magical formula that will work for every airplane installation.

Thanks,
Jimmy