In order for the FTE to generate correctly, the 'Contracted Weeks' should remain at 52 or else it should be left blank.
If the 'Contracted Weeks' is entered as 0 and the 'Standard Weeks' is 52, the FTE will automatically populate to 0.
I can see from the screenshot the contracted weeks for the employee is currently at 0.
If you change this figure to 52, the FTE will then auto-populate as you amend the hours.
See ticket 16855 as an example.