Need Measure to show behavior AFTER date from column

I've searched the forum and haven't come across close to what I need.

We have a program internally that insepcts customer activity. There were some customers that received a letter 6 months into the program to remind them of the action that is expected of them.

I'd like to write a measure that shows their behavior AFTER the date they received the 6 month letter. Since these customers are on different schedules, they wouldn't receive the 6 month letter at the same time nor period. 

I've created some sample data. The data is collected via a PowerApp that feeds into a SharePoint list.  There are two lists, one which is the landing spot for the PowerApp data and another that is more of a reference and contains all the customer information such as name, address, how many containers they own, if they received a 6 month letter, and what date the letter was sent to them. The SharePoint lists are connected to a PBI report.

What I need: 

- to show the closure rate of their bins AFTER the 6 month letter date.

This will require the use of the PowerApp data SharePoint list and the Customer info SharePoint list (6 month letter date resides here).

I don't know where to begin with it. 

This is the sample input data (link below):

This is the customer data

Here's the sample data, I couldn't figure out how to attach the files to the post so here's a GoogleDocs link:

Customer reference:  https://docs.google.com/spreadsheets/d/1GvFydlNvx3nWVngYzFpHJuJ6CXTt7V60/edit?usp=drive_link&ouid=103619957917641337701&rtpof=true&sd=true

Input data: https://docs.google.com/spreadsheets/d/1IqTXXBhF4pU55af_yS_OUh4jF46M2SHu/edit?usp=drive_link&ouid=103619957917641337701&rtpof=true&sd=true

Your help is appreciated.