Oh, I understand your question now. The IRS instructions for line 16 of Schedule AI say to incomed other taxes owed because of events that occurred during the months shown on the column heading. Include the same taxes used to figure Form 2210, Part I, line 2 (except self-employment tax), plus the tax from Form 4972, Tax on Lump-Sum Distributions; Form 8814, Parents’ Election To Report Child’s Interest and Dividends; and any alternative minimum tax (AMT).
It seems that the annualizations on Schedule AI are done under the assumption that your income, deductions and credits will accrue at the same average rate for the remainder of the year beyond the period for which the calculation is being done. With that assumption in mind, my guess is that you would need to annualize the investment income received through the end of the particular period by multiplying by 4, 2.4, and 1.5, calculate the NIIT based on that amount of annualized investment income, then include the resulting NIIT on line 16 of Schedule AI. This would be similar to the way that self-employment tax is done, again under the assumption that income will continue to be received at the same average rate for the rest of the year. It's certainly possible that that would result in Q2 being subject to NIIT while other periods are not, particularly if you received the bulk of your investment income in Q2.
