Below are all of the free child support calculators for NC. Granted, all of the child support calculators you find should be free. The ones below were created for you by NC. So, I'd follow these since it's what your state references.
You can always find and use other calculators. However, don't be surprised if you get different results.
One tip, remember that in most cases the amount paid towards child support can be shared. Considering how expensive insurance can be, definitely look into how it can help you reduce costs.