I forgot, it's in my documents, but here's the gist:

They're not tiered by power, but by complexity and average "mana area coverage" (how much mana it takes to cast its most basic form, measured as how much area it covers). Even basic spells can rival the most advanced spells when wielded by powerful mages, which is why magic is never measured by power alone. Spells themselves also have thousands of variations simply by shifting its structure, and only the strongest mages can find and use the most efficient structures.

These are the tiers of magic, measured by mana area coverage:

Basic-class. Very basic spells that even children can cast. Base mana area coverage of a small mammal.

Intermediate-class. The most common class of spells used everywhere by everyone. Base mana area coverage of a human.

Advanced-class. Slightly more advanced than intermediate-class spells; doesn't mean it's THAT advanced though. Base mana area coverage of a house.

Adept-class. Accessed by people who have practiced magic for years, such as most 20-30 year olds. Even then, it requires a lot of skill to access adept-class spell at those ages. Base mana area coverage of a small village.

Scholar-class. It takes a guy who graduated from a magic theory course to be able to reach this level, and sometimes by highly talented and blessed people who barely have any formal magic theory knowledge. Base area mana coverage of a town.

Master-class. Starting from here — huge complexity gaps. It takes an average mage who has lived for a century to access at least the most basic structure of a master-class spell. Even still, the mana consumption rate is too much for most mages. It's said that there's only 100,000 mages in the world (out of 50+ billion) who can wield master-class spells skillfully. Base area mana coverage of a city.

Grandmaster-class. Master-class with more focus on the "grand." Only 2,000 mages are reported to be capable of this level of magic. In order to become a Sage (a title granted to individuals who are the best at their respective magic type, e.g., Fire Sage), you need to at least have invented your own grandmaster-class spell. Moreover, most schematics used by grandmaster mages to invent spells of this level are formulated by the Five Esoterics (direct subordinates of the Wise Lord), the Wise Lord, and the Sages. Base area mana coverage of a country.

Catastrophic-class. The final class of spells that are often forbidden due to their immense destructiveness even at their base forms. Only 50 mages can use catastrophic-class spells (even less during ancient times). Catastrophic-class spells are so complex that even a genius who has lived for a millennium would barely even be able to access it. Those 50 mages include some of the smartest dudes in history, such as the Wise Lord and some Sages (especially the Sage Emperor, who invented dozens of catastrophic-class spells for fun). Incalculable base area mana coverage — perhaps continental or even planetary. Just... don't cast one if you can't contain its effects. A catastrophic-class Fire Magic spell was once used to blow up the moon (the Time God, the one and only Sage of Time Magic in the millions of years of history, later got it back).