One way you could accomplish this if the LetterNumber rank is likely to change often is using a segment and then a smart campaign to adjust the field value.
So you apply the correc...
You'll want to use the "Not Opened Email" filter.
If you want it for people that we sent/delivered a certain number of emails you can use it with the Was Delivered or Was Sent fil...
One method that I've seen work is assigning content to a persona/multiple personas, and then creating score fields for each persona to act as a counter. Each time someone interacts...