The basis was already reported on the 1099-B when the stock options were exercised and sold. Because this was a 'cashless' options purchase, the Employer included the gain on the sale in W-2 income also. 

I used the Capital Gains(Losses) Detail Entry Worksheet to record the stock option exercise and sales transactions and increased the basis by the amount already included in W-2 income on Line 11-corrected basis.

Many thanks to sjrcpa!