The amount that is deductible is determined entirely by your AGI without regard to the traditional IRA contribution. You can use TurboTax to calculate the deductible amount by entering a maximal traditional IRA contribution, then seeing the deductible amount that TurboTax puts on Form 1040 line 32 or Form 1040A line 17. Then go back an change your originally entered maximal amount to be the deductible amount instead.
If you are interested in the details of how the calculation is done, see this worksheet from 2016 IRS Pub 590-A but substitute the 2017 phaseout limits. The IRS has not yet published the 2017 version of Pub 590-A.
