62 1/2yrs old & retired. withdrawing $10K/mo expenses. Will i incur an underpayment tax penalty regardless of withholding % or not for a December large roth conversion?

You can't do anything for 2024. In 2025 here are the safe harbor guidelines:

The underpayment penalty will be assessed if any of these apply:

• "If you don’t pay enough tax through withholding and estimated tax payments, you may have to pay a penalty."
• "You also may have to pay a penalty if your estimated tax payments are late, even if you are due a refund when you file your tax return."

To avoid the under payment penalty:

• The amount you owe is less than $1,000, after subtracting withholding and refundable credits.
• You paid 90% of the tax that you owed for the current year.
• You paid 100% of the previous year tax, (110% for higher incomes).
• If your previous year's adjusted gross income was more than $150,000 you will have to pay in 110% of your previous year's taxes to satisfy the "safe-harbor" requirement.

"Typically, underpayment penalties are 5% of the underpaid amount, and they're capped at 25%. 

Underpaid taxes also accrue interest at a rate that the IRS sets annually."

To avoid this situation it is recommended that you adjust your W-4 with your employer or pay estimated taxes.

Underpayment of Estimated Tax by Individuals Penalty