Thanks for the additional thoughts.
I was aware that any taxes on my SS benefits should be considered in determining the amount of withholding I specify for my year-end IRA distribution. As my original post says, "But IRA withdrawals go into Adjusted Gross Income, which in turn goes into the formula that determines whether -- and how much -- federal tax I will have to pay on my Social Security benefits." My conundrum was determining the amount of estimated quarterly payments in advance of knowing the amount of the IRA distribution -- and thus the amount of SS that would be taxed. But with what I've learned here, that problem goes away because a withholding payment -- even if all made in December -- is considered by the IRS as spread across the year.
While I was also aware of the rule that underpayment penalty can be avoided by paying at least as much tax as owed last year OR 90% of the current year's tax -- whichever is lower -- I appreciate the reminders.
