Qualified Business Income (QBI): Spouse owns Sch C company. Income below QBI phase-out. QBI deduction calc'd by TT is much less than 20% of Sch C income (~4%). Why?

[DDollar]

It's really hard to say for sure without a lot more information, but it could be that the QBI deduction is the lesser of Qualified Business Income, or taxable income.  Taxable income is after other adjustments,  Itemized Deductions or the Standard Deduction, etc.