If my net business income is going to show a loss this year(nol), do I have to go back the two years and recalculate?
First, I assume you're talking about your business reported on F1040?
If that's the case, then just because you have a business loss, doesn't necessarily mean you have an NOL on your F1040. You may or may not. On a 1040 you have to combine the business loss with any other income included on your return to determine if your return has an NOL.
Assuming for the sake of argument you do have an NOL, you MAY WAIVE the carry back of the NOL to the prior two years. The election to waive must be made on the original filed return.
If you have positive income in the prior two years and paid tax, it might be to your advantage to carry it back and get a refund. On the other hand, if you anticipate large income in 2017 and beyond, the carrying the NOL forward might be better. It depends on your economic outlook.