The best place to start would be to establish your baseline by tracking CSAT at the end of each project. After 10-20 responses, you should have a good idea as to where you are at.

From there, I recommend interviewing customers who leave any scores that are below 4/5

So any passives (3) or detractors (<3)

Check out this article that goes over how to set up CSAT! https://help.guidecx.com/en/articles/6543345-csat-surveys