The short answer is yes, that is correct. Some of the distribution will be taxable but a portion will not be taxed.
The long answer is that a Traditional to Roth conversion is an IRA distribution subject to tax. As you have made nondeductible contributions to the Traditional IRA, the fraction of the distribution that is tax free is the ratio of the total of your nondeductible contributions in all your Traditional IRAs to the total 12/31/2018 value of all your Traditional IRAs.
Form 8606
is used for this calculation and makes an adjustment for the possibility that your transactions occurred in 2019.
Full details on contributions, distributions, and conversions may be found in IRS publications 590-A and 590-B:
https://www.irs.gov/pub/irs-pdf/p590b.pdf
