This is what I did, but it is a terrible oversight for TT Premier to leave this out. If you edit the form to show the amount not subject to NIIT, then you can no longer file electronically.
Also, if it had a way to indicate the NUA portion of a sale, it would also eliminate having to create two 1099-B forms if sold within a year of distribution (NUA is LTCG, appreciation since distribution is short term).
