Let me provide an example. Let's say the QCD you want to make is is $5k, the RMD is $10k and to avoid an underpayment penalty without annualizing income on Schedule AI you need to make up a tax-payment deficiency of $20k after figuring in your Roth conversion. Your husband could make a distribution of $10k with $5k being QCD and the other $5k withheld for taxes. Assuming that you plan to convert more than $15k, you could then take a distribution paid to you from your traditional retirement account equal to the amount that you intend to convert to Roth, have $15k of that distribution withheld for taxes, replace that $15k with other funds, then indirectly convert to Roth within 60 days the entire amount distributed from your traditional retirement account. Because conversions to Roth are disregarded with respect to the one-per-12-months rollover limitation, you avoid any involvement with that limitation. The amount withheld between the two distributions will be $20k, the intended QCD will be made, the RMD will be satisfied and you will have converted to Roth the intended amount with nothing left over.
The amount withheld for taxes (but not estimated tax payments) is subtracted from your tax liability before the remainder of your tax liability is divided among the four tax quarters . Estimated tax payments are then applied to the corresponding tax quarters to see if you have any quarterly underpayment. The effect is that tax withholding at any time during the year is divided among the four tax quarters, so tax withholding late in the year can make up for underpayment early in the year while estimated tax payments cannot.
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