in a tax-free merger, your basis in Dow is the same as your basis in DowDupont. Many taxpayers skip allocating any of the cost basis to the fractional share sold. however, if you wanted to the calculation would be as follows
cost basis of DowDupont divided by the shares (including any fractional shares received). this gives you the new cost basis per share for Dow. multiply this by the fractional share received and you'll have its cost basis
