Salesforce Customer Score Formula -- 3

I need a numeric formula added to a new customer case / report field in Salesforce. This formula will be used to calculate a customer score based on customer feedback.

This should be a very simple task to take one exisiting case fields (promotor or detractor depending on the customers feedback score) and add a new field that calculates the NPS score (tallying up your responses and subtracting the percentage of detractors from the percentage of promoters)

Requirements:
- The formula should incorporate customer feedback as a key factor.
- Customer feedback will be scored using a numerical rating system.

Ideal Skills and Experience:
- Proficiency in Salesforce, particularly in creating and modifying formulas
- Experience with customer score calculations
- Ability to integrate customer feedback metrics


NOTE: I have already cerated the drop downs and picklists, just need a field that will take that value and convert it to a NPS score.