The reply by @MinhT1 [now removed] is incorrect. A deductible IRA contribution will always reduce MAGI for the purpose of Medicare IRMAA. MAGI for Medicare IRMAA is AGI (Form 1040 line 11) plus tax exempt interest (line 2b).
(I think that MinhT1 thought that you were asking about the MAGI for the purpose of eligibility to deduct a traditional IRA contribution or eligibility to make a Roth IRA contribution, which is not what you were asking.)
