Thanks much for all the explanations and advice.
Based on most responses (and a little web searching I’ve done based on those responses), it appears that by ensuring my December IRA distribution has withholding equal to at least 90% of my tax for the year, I don’t need to make quarterly estimated payments at all. Which eliminates the need to predict how much of my SS will end up taxable before knowing the ultimate IRA distribution amount. And could also explain why I’ve never been assessed an underpayment penalty since doing year-end IRA withdrawals and withholding, despite making no estimated quarterly payments (I only started taking SS late last year).
While I picked @rjs’s post as the Best Answer, numerous others were also quite helpful.
