You can print out a copy of your return before filing it, which will include worksheets for how the qualified business deduction was calculated, but you can only print if you have pre-paid your TurboTax fees for the software you are using to prepare your return.

Please note that there are several components of the Qualified Business deduction and so it is rare for it to be a straight 20% of your business income.  There are two different income threshold levels where the calculation changes, but even for income under the first threshold the calculation is an "OR" as it is 20% of QBI (which is your qualified business income less your portion of self-employment taxes, self-employed health insurance, etc) OR 20% of your taxable income (except capital gains and dividends), whichever is LESS.

Here is more information on the Qualified Business Income Deduction, and included in the link is a high level overview section for how the calculation is determined in each threshold level.

https://ttlc.intuit.com/replies/7019998

Information on how to print your online return before filing:

https://ttlc.intuit.com/replies/3338601