Part 1. Use Line 16, Total Tax instead.
Part 3. Yes, to pay in quarterly estimates, using total tax charged would get you to 110%.
If you have the employer take out extra withholding within 2021 calendar year directly, then the payments are deemed to have been paid in ratably over the course of the year. There would be less chance of a quarterly charge.
For more information, see: How do I add Form 2210?
