You should contact a tax attorney or an accountant particulary one experienced in Mitchell Lama, HDFC or other limited equity subsidized properties that privatised. If the property was gifted or inherited pror to privatisation, I believe it is calculated from the new contract or if you were issued new stock or a deeed.
Most buildings that were limited equity or subsidized housing that became private usually have a "flip tax" (hence limited equity) A portion of your equity/gain usually goes back to the coop/condo. The amount you have to pay back to the coop/condo may be deducted from the basis as a cost of transaction or because it really isn't your gain.
The formula the coop/condo uses for their "flip tax" may help. If they use net profit that would be similar to calculating cap gain tax. The board or managing agent will probably know if you use the prior or post privatisation value.You really should seek tax and financial advice from a professional.
Mitchell Hall, Associate Broker
The Corcoran Group