The QBI deduction comes last. You do not need to factor in the QBI for your solo 401K deduction, but rather, the QBI deduction will factor in the 401K contribution in determining the amount of taxable income produced by your sole proprietorship for the purpose of the QBI deduction. Because of this, you can determine your 401K contribution like you may have done in the past on net income from the business (and taking into account the 1/2 of SE tax), but you do not need to factor in the QBI calculation for the sake of your 401K contribution.
I have provided a detailed answer on how the IRS is handling this, and it refers to the final regs just recently adopted by the IRS and Treasury Dept. It's detailed, but may help you to understand (There's also a link to a TurboTax FAQ that provides additional information as well). Please note: https://ttlc.intuit.com/replies/7161701
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