The broker's 1099-B should be reporting only your " out of pocket" cost to acquire the stock you're selling, i.e. (# of shares sold) x (exercise price per share). To that you want to add the compensation cost for the shares sold created by the exercise of the NQSO.
If you happen to be selling ALL the shares received in the grant, (we're talking about the GROSS number of shares stated in the grant, before shares are sold or withheld for taxes or exercise), then you could simply add the Box 12 compensation amount to the 1099-B amount.
In the more likely and more common situation where you are selling less than the GROSS number of shares, you add a the compensation element for only the number of shares sold.
So, for example, if the gross number of shares in the grant was 80 and the associated Box 12 amount was $640, then the"per share" amount would be $8. And if your sale was for 20 shares you'd add $160 to the 1099-B amount.
