Level 15

As a general rule, if the taxpayer Materially (or maybe "Significantly") Participated in the business, NIIT does not apply.  So you may be able to Google the rules about that.

It has been several years since I looked into it, but I seem to recall that you need to MANUALLY adjust the NIIT form if it involves the sale of a business that the taxpayer Materially Participated in.  Maybe they changed it since then, but just be aware it MIGHT require some MANUAL entry in ProSeries.