there is something else that i think is important. the ordering selection for the stock sold. if FIFO is the method you use for accounting for which shares are sold, then I believe the shares you sold were the ones acquired on 5/1.
The LIFO method is one that you have to elect affirmatively with your broker. The main benefit of the LIFO method is that the shares that you've owned for the shortest period of time tend to be the ones that have the smallest taxable gain, and so you can make a sale without incurring a large tax bill. if you elected LIFO then it was the 5/15 shares sold.
if it's fifo, then the broker that handled the sale won't know about the other shares and will, i believe, use the 5/15 purchase. this may require you to make manual adjustments from what is reported on your tax return with the appropriate coding so the iRS knows why what your reporting is different than the 1099-B.
