Define a set of criteria to evaluate on first, and then use that set to determine a final rating.
It can often be helpful to defer the final rating, and start with components. Many teams have characteristics or values that they are judging a group of employees on. In the below view, employees are rated on 3 values which then produces a composite score. Note that the Final Rating is not formulaically set, the composite score is seen as an input and then you can adjust to match.