Unfortunately, by rolling the 401(k) to a traditional IRA in the same year as you converted an amount only a bit more than your basis in nondeductible traditional IRA contributions, you've caused your conversion to be largely taxable. There is nothing you can do now to reduce the taxable amount of the conversion (other than recharacterizing the conversion back to a traditional IRA, something that will not be possible for conversions done in 2018 and beyond due to the recent tax-law changes). This is an mistake that many people make.
