MAGI is calculated differently depending upon the context. What are you referring to?
As Bsch4477 said, MAGI is calculated differently for different purposes. There are about a dozen different definitions of MAGI, depending on what it's being used for.
What do you need your MAGI for?
There are some situations where MAGI is used in a tax calculation but the calculation is done behind the scenes and the MAGI amount does not even appear on the tax form.
The additional deduction in the new tax law is not specifically for Social Security. It's for anyone who is 65 or older. The additional deduction is gradually reduced if MAGI is over $75,000 ($150,000 if married filing jointly) and it disappears completely if MAGI is $175,000 or more ($250,000 if married filing jointly). For this deduction, MAGI is Adjusted Gross Income (AGI), Form 1040 line 11, plus the Foreign Earned Income Exclusion and excluded income from sources in Puerto Rico, Guam, American Samoa, and the Northern Mariana Islands.