Retirement tax questions

A whole lot really depends on the $$ amount you converted, and the added taxes it eventually will cause.

Best option if possible:

1)  IF the extra tax amount is small enough, at this point in time, the best route (opinion) would be to increase your tax withholding at work by submitting a new W-4 with lower allowances...or a specified extra withholding amount with each paycheck, such that you get well-over the safe harbor level by the end of the year.  If done that way, there's no extra paperwork at tax time.  But like I say, it depends on the extra taxes you expect from the conversion.  And a new W-4 this submitted week, might not get started by your employer until the end of Sept paycheck.    But if enough can be withheld, this could satisfy the safe harbor requirements for you.

Less Easy Option

2)  Since the conversion was last January, the safe harbor level using estimated tax payments could have been accomplished, but that would have required either a large payment in the first quarter, or exactly equal estimated payments for every quarter to cover what extra is needed.   IF you have a large amount to pay that cannot be done with withholding...then you have done nether of these......but, starting your estimated payments now (one before Sept 15th and another before Jan 15, 2020) will potentially limit any penalties and interest for underpayment at tax time.  Even with these final 2 quarterly payments, you would likely have to prepare the Annualized Income forms  (2210/2201AI) at tax time to get the minimized penalty....those forms require that you enter  your total income from all sources based on which quarter they occurred, and all tax payments by quarter, and all deductions by quarter....etc...I find that to be a real mess to do as you need all your monthly statements from your bank/broker/employer...and detailed dates of all your charity contributions and state refund (if any) from last year.  I've done it just once...hope to never have to again. 

Either that you you just eat the penalty.

So option #1 would be better...if possible.

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Others here may have some other opinions, and I may have missed some detail that needs to be commented on.     If you decide to use estimated payments, try the make the first one larger before 15 Sept.  You can arrange an IRS Direct Debit a their website ( if you need to do state too...you'll have to check your state's tax website to see what they offer).  For estimated payments online, Don't wait until the last day...get it set to be debited by the 10-12th of Sept.

https://www.irs.gov/payments/direct-pay