Line 16 on form 1040 is the tax. Multiply your 2021 tax by 1.10 to determine what 110% of your 2021 tax would be. Subtract your estimated withholding from the 110% tax number to determine how much estimated tax to pay to have your total estimated taxes equal 110% of your prior year's taxes.
CORRECTION: Line 24 is the correct line for tax paid in the previous year. Unless your income is more than $150,000, you only need to pay estimated taxes totaling 100% of line 24.
