Madoka is doing what many, many people do. Comparing the extra, say randomly, 40 -actual- damage thunder gets from a potency buff as being more valuable than someone with less gear getting only a extra -actual- 20. But in math land, where stuff gets real. . . 20% is 20% is 20%, and 320 is 3.2x as strong as 100. The only hiccup is thunder extension from contagion beyond the RS buff duration, but that too can easily be mathed in both optimal and suboptimal number of ticks.