According to the description of your scenario you have two data sources: Table 1 and Table 2.
I suggest to take advantage of the merge feature in Tableau. This allows you to merge your two data sources using in your specific case user_id and week_id
Drag dimension Program to Row shelf, this make Table 1 as the primary data source
In Data, click on Table 2, your secondary data source and make sure it is linked with Table 1 by clicking on link symbol to the right side of user_id and week_id
Create your calculated field % Reach as COUNTD(user_id)/COUNTD(Table 2.user_id)* 100
Drag the calculated field %Reach to text
If you share a package workbook with sample data, it would be easier for us to find a solution and illustrate it for you.