Generally, you base the estimated tax on the self-employment portion of income. Please see the page at the IRS website below:
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
However, you should consider your total income to get a more accurate estimate:
When attempting to estimate your income tax for the year ahead, you are predicting the future. If you underestimate, you may be penalized for the number of days it remains unpaid. The simple way to ensure that you pay what you owe is to pay at least 100 percent of the tax you paid the previous year, unless you have some indication you are going to earn significantly less.
If you think you are going to make less, calculate about how much and try to pay 90 percent. If you pay 100 percent, and still owe a little more at the end of the year, that’s OK.
There are "safe harbor payments" — a payment that ensures you will not be penalized. For example:
If you are married, filing jointly and your adjusted gross income is below $150,000, you may make a payment equal to 100% of what you paid in income taxes the previous year or 90% of the tax you estimate for the current year.
Taxpayers whose adjusted gross income is $150,000 or more must make a payment equal to 110% of the previous year’s taxes or 90% of the tax for the current year.
