Handling division with slicer

Hi,

This should be straight forward but I've not been able to find an obvious answer yet - I have an import from Excel of historical monthly performance data, which looks like a more complicated version of the attached:

I want to make a visual displaying the calls, sales, and percentage of sales:calls for each worker individually and for groups of them, and I'm having an issue getting this percentage displaying properly for multiple workers (sums work fine). I'm calculating it with a simple new column Percentage = SheetName[Sales]/SheetName[Calls]. If I put a slicer on and display just Bob, it correctly shows 50% for January, similarly for Andy it will show 100%. If I select all workers, I want to be able to have it show 85% (i.e. 17 sales from 20 calls), but I can't see how to do this, the closest thing being an average of the column, which incorrectly returns (100+100+50)/3. Can anyone suggest what I'm doing wrong and how to get the output I want?