Ok, so the calibration grid definitely relies on the values in 7408/9 so you should focus your efforts on ensuring these are populated.
How are you capturing/assessing performance and potential?
For Potential (7408) are you using the Talent Assessment process and is that completed for the CPs in question?
For Performance (7409) are you using SAP (on premise) Perf Mgmt? Are the forms complete & approved?
Once complete (or closed approved if the approval process is activated) then follow up processing gets triggered (uses BAdI HRHAP00_FOLLOW_UP). This should then populate HRP7409 for that CP.
Stephen