You can try this, not sure whether it'll work or not
Create a calculated field like this
(SUM([WorkedHours]) + SUM(IIF([Assignee] = [QAAssignee] , QATime , 0)) / SUM([AvailableTime])
Drag the date with datepart() Year, Month and Assignee then put Usage % on label or rows/columns shelf in canvas and check
Else share some sample mock up data in packaged workbook so that any one of us can take a look and guide you to get the correct output.