Calculating a Percentage Based on Filtered Item Type Counts

I am attempting to calculate a percentage based on a total task count sorted by total item types. The logic is something like this:  (ItemType1+ItemType2+ItemType3)/(ItemType4)=%. 

Basically, I am trying to calculate an error rate. Tasks in item types 1-3 are production units, whereas tasks in item type 4 are rework units. I would like to return this percentage for each employee. Here is my current set up. What am I doing wrong?