I'm sure you guys would figure it out at some point but for convenience sake...
Muggle - Less than 5 magic
Squib - 5 to 50 magic. Not enough magic for spell work but enough to imbue intent into writing for written magic, even if they can't activate them on their own.
Tier 0 - 51 to 10,000 magic with a level 1 core. Normal, everyday Magicals are born at this level.
Tier 1 - 1 to 10,000 magic with a level 2 core. Normal, everyday Magicals graduate standard magical education at this level and all but the weakest children of demigods are born at this level.
Tier 2 - 1 to 25,000 magic with a level 3 core. This is as powerful as most people get without excessive training or a lifetime working a magic intensive job.
Tier 3 - 1 to 25,000 magic with a level 4 core. This is the ceiling for people with average talent and no bloodline, without significant outside factors.
Tier 4 - 1 to 50,000 magic with a level 5 core. This is the limit for 99.9% of people who pass tier 3 and the mortal ceiling.
Tier 5 - 1 to 100,00 level 6 internal magic, 1,000,000 divine magic. Reaching this level makes you a demigod.
Tier 6 - 1 to 100,000 level 7 internal magic, 5,000,000 divine magic. Notable tier six mages from Earth include, Albus Dumbledore, Gellert Grindelwald, and Tom Riddle Jr.
Tier 7 - 1 to 100,000 level 8 internal magic, 50,000,000 divine magic. Notable tier 7 mages from Earth include, the 4 founders of the original Hogwarts and Nicolas Flamel.
Tier 8 - 1 to 100,000 level 9 internal magic, 1,000,000,000 divine magic. This is the demigod ceiling. Notable tier 8 mages from Earth include Perenelle Flamel, Mary the Jewess and Hassan-i Sabbah.
Tier 9 - Energy body, 1,000,000,000,000 base divine magic. Reaching this tier makes you a god. Notable tier 9 mages include Merlin, the current Thor and the previous Apollo and Artemis.
Tier 10 - Energy body, potentially infinite divine magic. No tier 10 mages from Earth. Notable tier 10 mages include any Sky-Father or comparable mage.
Notes: Demigods no longer have a core and just saturate their body with magic. 100,000 is the most magic a body can hold. Every magic level is a 5 fold increase in magical density.