For an explanation of Modified AGI, please see this TurboTax Help article: What Is the Difference Between AGI and MAGI on Your Taxes?
If you would like to calculate your MAGI, you can use the worksheet listed on p. 40 of IRS Pub. 590-A Contributions to Individual Retirement Arrangements (IRAs)
The page 40 worksheet in Pub 590a references the OLD 1040 form and does not match the new, shortened 1040 form.
Your total income is on line 7b of the new form. Then Schedule 1, which has IRA contribution, goes on line 8a. The difference creates line 8b for AGI.
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