Your self-employment tax cannot be lowered by claiming the QBI deduction. This is due to the QBI deduction not reducing your self-employed income that is reported on your Schedule SE.
The QBI deduction is a subtraction to your taxable income that is reported on your 1040 form. This deduction can be thought of as an additional deduction to your itemized or standard deduction. Your QBI deduction and itemized/standard deduction are listed before your income tax is calculated.
The self-employment tax is added, additionally to your income tax, after your income tax is calculated.
For more information, please review the IRS web-page, Facts About the Qualified Business Income Deduction.
