I need my current year agi to know how much i am limited in contributing to my IRA, where can I find the estimated MAGI before I file my taxes?

turbotax says I have to look on my 1040 to find my agi, but I need a MAGI to calculate my maximum deductible roth contribution for the year before I make the contribution, and then report the contribution when I file, so how do i get an accurate estimation of MAGI before I file?