"The instructions say that IRA contributions have to be added back"
No, not for Medicare IRMAA. MAGI for Medicare IRMAA is AGI plus tax-exempt interest (Form 1040 line 11 plus line 2a).
(You appear to be looking at the calculation of MAGI for the purpose of deducting an IRA contribution or for making a Roth IRA contribution, which are an entirely different MAGIs.)
I'm not aware of any MAGI that requires adding back the self-employed retirement deduction.
There are dozens of different ways to modify AGI for different purposes. For what purpose do you need to determine MAGI?
In this case, I am trying to see if my Medicare premiums will go up because of an increased MAGI. The instructions say that IRA contributions have to be added back, but are silent regarding whether SIMPLE IRA contributions are considered IRA contributions or salary reductions, such as 401(k) contributions. 401(k) contributions are not added back. I can't find my 1040 MAGI calculation anywhere in my Turbotax file. I need to know this to see how much of my income I need to defer to keep below the MAGI level. It can save me thousands of medicare premiums annually.
"The instructions say that IRA contributions have to be added back"
No, not for Medicare IRMAA. MAGI for Medicare IRMAA is AGI plus tax-exempt interest (Form 1040 line 11 plus line 2a).
(You appear to be looking at the calculation of MAGI for the purpose of deducting an IRA contribution or for making a Roth IRA contribution, which are an entirely different MAGIs.)