The instructions for line 42 (2015) can be found on the worksheet on page 41 of the pub below:
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-prior/i1040gi--2015.pdf">https://www.irs.gov/pub/irs-prior/i1040gi--2015...>
There's not much possibility that Turbo Tax made a mistake with this, I have to say. Its a basic calculation.
