Incidentally, my reading of RAW is that Mastered Ritual spells have no botch dice when cast under calm conditions, because the description under Ritual spells says "always use a stress die", not "always count as stressful". I suspect that that was my intention at the time, because a number of ArM4 to ArM5 changes in that section were designed to create a background where magi could use magic to do casual, everyday stuff without statistically exploding with unnerving frequency, but it was over ten years ago and I don't remember exactly what I was thinking at that point.