If your goal is to create an income tax liability that equals (or at least comes close to) your otherwise unused credits, it's best to prepare at least an estimated tax return. That can be a bit tricky to do before year end, but it doesn't *have* to be done by year end. You must make the distributions from the traditional IRAs by December 31, 2017 for the income to be taxable on your 2017 tax return, but you have 60 days to complete a Roth conversion or a rollover of the distribution. You could potentially take distributions from the traditional IRAs a bit in excess of the amount that you need to convert, complete your actual tax return to determine the optimal amount to convert, complete the conversion of the optimal amount, then roll the remainder back into the traditional IRA, all before the end of the 60-day window. This is a bit riskier than just doing the conversions outright because you can potentially get tripped up by the one-rollover-per-12-months rule (per individual) on the rollover of the remainder back to the traditional IRAs.
