You can create a calculation like the one below that brings in numbers from both data sources:
SUM([Count])/SUM([Total Installs].[Total Installs])
I've attached a workbook that shows this example. With this method you can keep a live connection to the Total Installs number while extracting and refreshing your Cases on a schedule.
Let me know if this answers your question.
Blending_Dividing.twbx 25.9 KB
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The secret to Keshia's example is in NOT having a blend relationship defined between the two data sources. (You'll notice when you open her workbook that Tableau gives you a warning about that. Sometimes you need to do that, warning or not!)
In fact, Tyler, you can even create calcs in that secondary source that key off of parameters (for instance start date and end date) so that you pare down the rows you want to use from there. SUM([secondary].[calc field]) instead.
Beautiful stuff that gives you magical powers!