How is MAGI calculated for Roth IRA

TurboTax adds back my partnership losses to my AGI to figure out MAGI.  This seems to be correct, as it's described in numerous places on the intuit site, including here: 

https://turbotax.intuit.com/tax-tips/irs-tax-return/what-is-the-difference-between-agi-and-magi-on-y...

 

What I want to know is why the IRS publication doesn't mention this in its own worksheet, found here:

https://www.irs.gov/publications/p590a#en_US_2019_publink[phone number removed]

 

Does TurboTax have some other source for this calculation besides the IRS publications?  How can I know this is correct?