Single Overall Rating for Performance Reviews

Summary

We would like the employee and manager to give one single overall rating on the self-assessment and performance review.

Content

We currently have the employee self-assessment where they give ratings on each competency and then calculate the over-all rating, the same for the manager when they do the employee review.  We would like to change this to just one overall rating in the header of the review and no individual ratings in the competency sections but cannot find how to do this.  Can someone please assist?