How can we restrict Managers to select rating in Overall Summary section

Summary: Based on the Calculated Rating in Goals Summary section, we want the Managers to be able to choose the Overall Summary Rating one level up or one level down and no other values. How can we achieve this?

Content (required): Suppose, the Calculated Rating is 3.5 for Goals section based on individual goal rating multiplied by the goal weightage and the sum of them, the Calculated Rating is considered as 3. Now, we must put a restriction as such so that the Manager can capture the Overall Summary rating as either 3 or 2 (and not below 2) or 4 (and not above 4).

Can we achieve such restriction? If yes, how?