My views on spell mastery of rituals and botches are rather outspoken and start here. RAW is ambiguous and contradictory. In that debate, I can find language in RAW that suggests that rituals, as long as they aren't considered stressed, may be cast without a chance of botch (page 83, the last sentence in the section that started on page 82 on Using Raw vis) . I find the idea of handling vis to be always stressful, which is what I'm basing my interpretation on.
I believe that rituals are always cast under stress and thus subject to botch. This is one thing that makes Mercurian quite valuable, since they cut the chance of botch in half, since by raw it is definitely a double edged kind of virtue. Consider the Mercurian Cautious sorcerer with a point of mastery in a ritual spell: they can cast an 8th magnitude ritual without any risk of botch. 8 pawns of vis becomes 4, cautious sorcerer removes 3, and the mastery removes the final. Adding a familiar and a strong Golden cord will just push that up. Finding a familiar and making a golden cord of 1 changes the equation to a 10th magnitude spell, 2 is 12th magnitude 3 is 14th magnitude. Add another point of spell mastery and you get someone who can cast Hermes Portal without any chance of botch.
That being said, given that this is a simulation, sticking to a strict interpretation doesn't mean anything. Die rolling isn't something that I think is really necessary for this. If you want to do it for a story for an element of unpredictability that's cool, I'm gearing this more towards interesting stories and characters. In my view, the rules will shape questions of risk and how a participant responds to the risk, that's where there are interesting stories. For example, Ysebrand, my my failed apprentice with a Hunger for Corpus. His story is interesting and based on the premise that his former master tried to improve his Stamina with a ritual. The ritual was botched, destroyed his Gift and made him forever hunger for Corpus vis. His story is just beginning, but I have a plan for him. The maga I'm running in that saga, Talia of Tytalus, also has an unusual background, but not quite so strange as Ysebrand.