Need help with personalization of a DFF enabled in Performance document

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I have enabled a DFF in performance document to track "% Personal Goal Completion" . As per customer requirement, this has to be a mandatory field for manager. So I expanded the additional region and then put EL expression to make the DFF mandatory for manager. Now as per functionality, manager will put this value for employee after looking at system calculated rating. But system is triggering this mandatory validation  when user tries to click on calculator button. Is there a way to trigger this mandatory validation only when employee click on submit /save button.

Note : We thought of using questionnaire for this requirement but we have couple of validations (like it has to a number without decimal and between 0 to100) for this field  which can not be achieved using questionnaire. through DFF, those are easily achievable.