It’s counter-intuitive, but when you’re writing about magic, you need to make sure it’s consistent. Magic is a chaotic force, defying physics and logic. However, magic has to follow rules, otherwise your readers start getting dubious about it. If your hero can throw a fireball which kills a werewolf, then the villains can do the same, unless there’s some reason they can’t. The same applies to other supernatural elements. How much can that werewolf bench press? How fast can a vampire move? If these factors change randomly, people notice and the suspension of disbelief necessary for a fantasy story is broken.
So, you write down the rules of your magic. You write down how werewolves shift and how strong they are when they do. How it’s done is the choice of the author, but it’s always a good idea to have notes and rules about how everything is going to operate.
I keep my notes in Microsoft’s OneNote, which lets me have file folders I can access from almost anywhere, even on my phone. Some of these notes are narrative, generally those involving character and world histories. Since I’m an old role-player, I use role-playing game rules to write-up the world’s rules and what the characters can do. In this case, I use GURPS, the Generic Universal Role Playing System from Steve Jackson Games. All the major characters have write-ups in GURPS 4th Edition. The magic system(s) are written up using GURPS as well and are heavily modified versions of the GURPS Magic system. I do cheat; narrative gets to trump exact application of the rules I’ve constructed. Generally though, I stick to my rules so that I know what can happen in a given situation.
GURPS uses a point-based system for allocating attributes, abilities, and skills. The total point level of a character gives a somewhat objective analysis of their relative strengths. In case you’re wondering, in the three published Thaumatology books, the most powerful character to appear so far is Faran, Lily’s father. And Lily is second. Remus is up there with them, approximately. Ceri has significantly less raw power than Lily does, but Ceri is far more flexible. It’s Ceri’s flexible magic which gives her the edge over others who have more sheer power, but in specific areas. And Michael can bench press 360 lbs without breaking a sweat.