Assume I convert $200K to a Roth IRA in January 2019. I estimate that my total tax liability for 2019 will be $40,000 (including the converted amount). Because my annual income will be "front-loaded" in 1Q, do I need to make an additional tax deposit in 1Q, or can I make quarterly tax deposits of $10K equally throughout the year?
Technically, you are supposed to pay tax as you earn income, i.e. you owe the tax fully in the quarter that you receive the income. However, I haven't noticed any problems with the IRS, and I do exactly the same as you. I withdraw the RMD and the Roth conversion amounts in January also. (make sure you take out any RMD before you convert, if you are over 70.5).
There is a way to treat this in TurboTax. At one point you are asked if you want to "annuitize" the income. If you say yes, you are asked to present your income on a quarterly basis. My advice; try it both ways, and select the one you like better.
Your suppose to pay each quarter whatever became taxable in that quarter. So in the first quarter, if you do the conversion you should also pay the tax on the conversion in that quarter. But the IRS also allows the following which is an exception to that rule. If you pay during 2019 timely quarterly payments that equals 110% of your total 2018 tax liability, then there is no penalty due when you pay the balanced owed by April 15, 2020
The following should still apply for income > 200k
Estimated tax safe harbor rule. If your 2016 adjusted gross income (AGI) was more than $150,000 ($75,000 if you are married filing a separate return), you must pay the smaller of 90% of your expected tax for 2017 or 110%* of the tax shown on your 2016 return to avoid an estimated tax penalty.
You can make 10K quarterly estimated tax payments. You can use the safe harbor rule to check if the payment is adequate to avoid a penalty:
2019 tax withheld + 2019 estimated tax > 100% or 110% of 2018 tax (depending on income)
If your 2019 income is more than 2018, you may owe more tax when you file your 2019 return.
If your 2019 income is less than 2018, you may get a refund.
If I understand correctly, then I should be able to perform the following in a somewhat reverse scenario and be OK.
If I am receiving some rather large taxable distributions in December that will cause me to owe considerable more tax, then I should be able to make an estimated payment to cover them in the 4th quarter only with no consequences. It makes sense, but you never know with the IRS. I have been doing considerable extra withholding, but this year it appears it's not going to come close to covering my tax liability.