Copy the manual section rating from calculated rating and make it read only for manager in PD

We need that manual section rating on overall summary section in performance document should be made read only and line manager should not be able to edit it. Since, we need to do calibration in Oracle, we would like to keep this field enabled but non editable for managers on the performance document. Is there a way we can achieve this?

Please let me know if we have a way to achieve this