You were estimating, you just weren't estimating enough.
When you do withholding, you are directing that a certain amount of money be put into your account at the IRS to be held pending doing your tax return and finding out how much money you really owe. That is, the withholding is not actually paying the tax you owe, because you don't know how much you owe yet.
When you worked, your employer had IRS withholding tables and a W-4 to make a good estimate of how much you would owe at the end of the year, so for most people it works well.
But for retired people, you don't have that aid.
Instead, you need to develop an idea of how much tax you will owe at the end of the year, then set your withholding to be one twelfth of that each month. Personally, I would make it slightly larger than that because of of COLA increases and the like.
And if you take an IRA distribution, then you will need to set the withholding to cover what your additional tax will be when the IRA income is added to your return.
Normally, I do this by getting an idea of what tax bracket I am in. If I am in the 12% tax bracket, then I need to withhold 12% of the IRA distribution - unless the IRA distribution is so large that it will put me into another tax bracket.
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