Whenever she gets called on for help (like, literally: in one case she was fending off a dybuk possessing a woman but didn't have access to her Guardian magic until one of the possessed's friends said "...help her!") she has access to like Blackstaff level magic. It can only be used to help someone or take out the Threat Against the Innocents (another example: in the book "Burning Water" she fought an angry Aztec deity for an innocent woman, and won).
In game terms, I'm thinking, she has thaumaturgy "on her own" but Guardian Magic is evocation but she only can access that if someone literally asks her for help. I just don't know how that would work. Make it a Catch? But then what cost would it be?

That I'm using? The "Dresden Files Character Manager" It's basically a program that fills in things for you. For a custom anything, though, it's not going to be worth trying to beat the program into submission.

"I shall not answer "Pure Freaking Magic" when asked how something happened."
Although it is true it is not helpful.