Amount of your reduced Roth IRA contribution. If the amount you can contribute must be reduced, figure your reduced contribution limit as follows. 1  Start with your modified AGI. 2  Subtract from the amount in (1): 3  $183,000 if filing a joint return or qualifying widow(er),f the amount you can contribute must be reduced, figure your reduced contribution limit as follows. 1  Start with your modified AGI. 2  Subtract from the amount in (1): 3  $183,000 if filing a joint return or qualifying widow(er),