It depends on what you want to do with it; speed or power? That's your choice. Follow the suggestions for testing in the article to fine tune it. It's all a matter of whether you are getting the performance you want out of the prop. If you find yourself bogging down, you need to go for a prop with more power, but you sacrifice speed. If you want to go faster and don't load your boat down with a lot of people very often, you can go for a higher speed prop as described above. Every boat has it's own characteristics depending on hull shape, engines and the load it's carrying. You do have to do the testing bit. There's no off-the-top-of-your-head answer. It depends on your boat and what you want to do with it. Boats are like wives. You have to learn their idiosyncrasies if you want to get along with them.