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Looking for the best formula to compare two sets of numbers

Set #1 - Rank: Smaller numbers indicate higher "rank" (Ex: 1 is the best, 1,000,000 is the worst)

Set #2 - Number of Items: The more items in a category, the worse off the category is

This formula is essentially for profitability, and is relating to Amazon Kindle Books

Here's some background about how Amazon ranks things (set #1)

For each category, there's roughly 10 best sellers that can be pulled, sometimes less if the category is smaller. For each best selling book, it's assigned a SalesRank (#1 is the best-selling book)

  1. I counted how many best sellers there were for a category, then took each of their SalesRanks and averaged them (is there a better way?)
  2. I then counted how many books were in each category.

So, what I'm looking for is a formula (preferably spitting out on a 1-100 scale) that finds categories with high sales ranks, but a low number of books in the category

Thanks in advance for any help from brilliant minds, and let me know if you need more info!

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