best to dissect the impact of the tax law changes from withholdings ......not aware of any changes to IRAs in the tax law change
1) take out the 2017 Form 1040 and divide line 63 (total taxes) by line 37 (adjusted Gross). for 2018, on Form 1040, divide line 15 (total tax) by line 7 (adjusted Gross). Assuming relatively similar income from one year to the next, that percentage difference is the impact of the tax law change
2) take the difference of the resulting percentages and multiply by the 2017 Form 1040 line 37 and THAT is the dollar impact of the tax law change against last year's income. For example, let's say the percent was 10% last year and 11% this year and the Adjusted Gross was $50,000 last year. then the impact of the tax law against last years' gross was $500 ((11%- 10% * $50,000)
3) take the difference between what your refund was last year and subtract what you owe this year and then subtract the result from #3. that is the impact of withholdings. For example, if last year's refund was $300 and you now owe $1,000 then that difference is $1300. then if the result of #3 was $500, then the tax law caused you to pay an extra $500 and changes in the withholding tables drove the other $800 ($1300 - $500)
a lot of math, but it can help understand what is tax law driven and what is withholding driven
