You’re in luck on the Thaums: Azalea has written about capacity on these forums, including the AMA:
Plus, its the subject of much speculation on readers’ parts. If you do a search, you’ll find a lot about it.
I’m guessing the weakest beginner might scrape together only 3-5 thaums. And, everyone seems to have a trick or three, and the spark shooting spell (first book) is less than ten thaums:
the average of ten among true novices …
I guess the real unanswered question has been, how many thaums for a big spell? All we know for sure is true invisibility will kill you, and Lacer manages it with probably over 4000 thaums.
On to potions:
I think they are a lot more dangerous than S. lets on. I think she easily could have hurt herself badly with the darkness potion, and lucked out.
But, people look at RQ in wonder because she is also an incredible potion maker. She read a couple of books on potion making and came up with a potion so interesting the Red Guard would like the recipe.