TurboTax implements Worksheets 1, 2 and 3 of Appendix B of IRS Pub 590-A to calculate modified AGI, the traditional IRA deduction and taxable amount of Social Security benefits, respectively, when you are subject to IRA contribution deduction limitations and you receive Social Security income. You can view these calculations in forms mode of the CD/download version of TurboTax by opening the IRA SS Wks.
https://www.irs.gov/pub/irs-pdf/p590a.pdf
