You can use table calculations to find the % of total.
First I Filtered out KPI = FTE as this is not needed. The I right clicked on the Sum value and selected the quick table calculations - Percent of Total:
By default,tableau may calculate the % of total for the whole. Right click on Sum value again and edit the table calculation to be calculated pane down. This way you're working out the percent of total per department:
Final result should look like this:
Please see the attached workbook for the example.