Get your taxes done using TurboTax

There are underpayment penalties imposed unless:

The total of your withholding and timely estimated tax payments didn't equal at least the smaller of:

1. 90% of your current year's tax liability, or

2. 100% of your previous year's tax liability. Your previous year's tax return must cover a 12-month period.

* If your adjusted gross income (AGI) for the previous year was more than $150,000 ($75,000 if your previous year's filing status was married filing separately), substitute 110% for 100% in (2) above.

Also, there is no underpayment penalty if the total tax shown on your current year return minus the amount of tax you paid through withholding is less than $1,000.

Specific answers to your questions:

1. The safe harbor rule you mention applies whether you increase your withholding to 110% of last year's tax OR make estimated payments to get to the same amount
2. The "total tax" from Line 24 of 2023's tax return is the one that matters
3. Underpayment penalties are calculated "per payment" -- essentially for each quarter individually. Withholding is generally treated as having been made throughout the year, regardless of when it is actually withheld. You could also make an estimated payment for the quarter in which you made the Roth conversion.
4. There isn't a specific maximum set by the IRS in the instructions for Form W-4 (line 4c for additional withholding). But your employer or payroll company may have a restriction.

You can make an estimated payment to the IRS at any time by using the Direct Pay option at IRS | Payments or by creating an IRS account and making a payment there. [Having an IRS account is extremely useful and worth the bother of verifying your identity to get it set up.]