Box 1a ordinary dividends is taxed differently than box 2a capital gains. So that would not work. It would also result in a mismatch from the 1099-div submitted to the IRS.
There must be a calculation to determine a 50% aggregate adjustment to all the income in boxes 1a, 1b, and 2a. But nobody seems to know what it is. And yet the IRS only provides one box for one aggregate adjustment so there must be a calculation!
