Yes, you would start with Line 24 (Total Tax) which will include your projected "Self Employment Taxes". Then you could adjust for any withholding you expect during the Tax Year. (W2 for spouse, any SS withholding etc.) This should give you a reliable figure as base for your estimated tax. Keep in mind, you must pay the lower of 90% of the tax shown on the current year’s return or 110% of the tax shown on the return for the previous year.
Below are links to IRS and Turbo Tax that have helpful worksheets.
Estimated Taxes | Internal Revenue Service
Calculating estimated taxes
The calculation is based on an estimate of current income. To help with the estimation, you can start with the previous year's federal tax return. Look at the taxable income, tax paid, credits and deductions from the previous year and compare to the current year’s numbers. The Form 1040-ES package includes worksheets to help you account for differences between the previous and current year’s income and calculate the tax you owe.
Generally, an underpayment penalty can be avoided if you use the safe harbor rule for payments described below. The IRS will not charge an underpayment penalty if you pay at least:
- 90% of the tax you owe for the current year, or
- 100% of the tax you owed for the previous tax year.
This rule is altered slightly for high-income taxpayers. If the adjusted gross income on your previous year’s return is over $150,000 (over $75,000 if you are married filing separately), you must pay the lower of 90% of the tax shown on the current year’s return or 110% of the tax shown on the return for the previous year.
However, if you do not pay at least that much via quarterly estimated payments, you may be subject to an underpayment penalty.
Your state will also have estimated tax payment rules that may differ from the federal rules.
