Qualified Business Income is calculated on taxable income attributed to the business, and not strictly on net profit of the business.  Since both Schedule C (sole proprietors) and Schedule F (Farms) are subject to self-employment tax, a taxpayer reporting a Farm on their personal return will have a deduction for 1/2 of their SE tax they pay in.  The IRS has adopted the position on their Final Regulations for QBI that this deduction reduces the amount of income that qualifies as QBI.  In addition, if you claimed self-employed health insurance deduction or retirement contributions that were made because of the business (such as a solo 401-K), these are also attributed to the business and reduce QBI.  

There is another factor as well.  In addition to calculating what the QBI deduction is on taxable income of the business, the QBI deduction is also calculated on all taxable income, and your QBI deduction is the lower of the two.  If all of your income came from Schedule F, your QBI calculation can be even lower.

Example:  Your Farm produced net income of $100,000 in the year, and you are Married Filing Joint and have no other income.  With 1/2 of SE tax and a self-employed Health Insurance Deduction, the taxable income from the business is 80,000.  The QBI calculation on that amount is 20,000.  However, since you have no other taxable income to report on your return, your final taxable income is 56,000.  20% of that amount is $11,200, and that would be your QBI deduction.

If you wish to, here's a link to an answer I provided which gives more detail on why the IRS is calculating QBI on your income the way they are (it's a bit lengthy but has valuable information from the Final Regulations to assist you:  https://ttlc.intuit.com/questions/4590833