How to display a calculated percent of 2 distinct counts on a dashboard

[kelsey_leerkes-]

How would I create a KPI on a dashboard this is a calculated percent of 2 distinct values.  For example, if you had KPI that was a distinct count of Invoices, and then another KPI that was a distinct count of Invoices that included a certain type of items.  A tile displayed all of the invoice details with filters to be able to only include certain types of items.  I need to display a third KPI that indicates what percentage of invoices has the types of items included in the filters.  Essentially the calculation is:
         count(distinct( invoice_number where  filter items exists)/count (distinct Invoice_Number)

I can get these 2 different measures in the same Look, but can't figure out how I would then use them to calculate the percent.  

[idaok]

Unless I'm reading it wrong, it would be a table calculation written in the same way you would mathematically or in excel.