You assumption is correct, the basis remains the same but becomes a smaller percentage when you take a distribution. This, of course, assumes your 401K and defined contribution pension had no after tax basis, that was also rolled over.
But, you did not need to consider the 401K and pension, when you did conversions in 2016 and 2017, because that money was not in an IRA, at that time. IRAs, pensions and 401ks are taxed separately. You may have heard that you must aggregate all Traditional IRAs (TIRA), including rollovers, when you do a conversion. That does not mean all tax advantaged retirement accounts. It only means TIRAs.
