Matrix measure between two tables common value includes multi Slicers

Hi!
It sounds like you should get your result using just a few measures. I'm not exactly sure how your March Retail table is structured so you might have to adjust accordingly, but something like this.

Sum of Sale Count := 
SUM('March Birthday'[Sale Count])

Sum of Sale Country Total := 
SUM('MarchCountry Total'[Sale Country Total])

Sale Count divided by Sale Country Total :=
DIVIDE(Sum of Sale Count, Sum of Sale Country Total)

 
This is using three separate measures. You could do everything in a single measure, but it's usually considered good practice to create separate measures for basic measures such as "Sum of..." since they often can be reused in several other measures.

Hope this helps!