Setting up NPS Net Promoter Score Calculation

I suppose setting up a workflow rule would do the trick nicely - but you will probably need to have two separate fields.

Yes, best practice would be to use 2 fields.  I would use a scheduled workflow rule to convert all of the existing data and a workflow rule for new NPS surveys going forward.  

Use can use a nested IF statement or CASE function.

https://success.clarizen.com/hc/en-us/articles/205712048-22-2-Clarizen-Functions?search=case+function

As an example, set the 0,1,-1 field using CASE($C_NPSfield, 0, -1, 1, -1, 2, -1, 3, -1, 4, -1, 5, -1, 6, -1, 7, 0, 8, 0, 9, 1, 10, 1). 

What do I have setup wrong?

The function needs to begin with Case(

If you double click Case on the right panel it will add an example of the function. Also, the custom field should start with a $.  

Case ($C_NPSScore, 0, -1, 1, -1, 2, -1, 3, -1, 4, -1, 5, -1, 6, -1, 7, 0, 8, 0, 9, 1, 10, 1)

Ah, you also need to put the numbers in quotes, forgot about that.  "0", "-1", "1", etc..  Let me know if that works, thanks.

Perfect, thank you!