It's also important to remember that most engines will actually run fine with a large range of props. The "best" prop always depends on the engine, airframe, air density (ie flying in denver) and how you want to fly.

The forumlas (like twice the diameter plus pitch) are rough guides, but really don't tell you that much. They do keep you from putting on a prop that is way to big or way to small though. In both cases, you can damage your engine.

To complicate things even more, different brands of props act differently in the air. For example, an APC brand prop will fly noticeably different from the black MAS at the same size.

You should do the search mentioned, props are a common topic