Thanks maglib. The question goes to a 2017 1099-R that I just received now. The conversion event (covering two years worth of contributions) occurred in March 2017, which would be fully reflected in my 2017 return as it was a 2017 event.
However, looking back, it does look like my 2016 Form 8606 reports basis for last tax year (I had been assuming I had reported 0). So that solves it for me, at least.
