Hi. I hope no one minds that I haven't qualified for membership yet, but I saw the thread title and wanted to share a thought:

Instead of thinking of numbers, think inside (towards the hub/BB) and outside (away from the hub/BB). Inside = slower and easier; Outside = faster and harder. As long as you avoid cross-chaining, you can use any combination you want to get the gear ratio you need for the conditions.

That's it. Don't over think it.