If we want to make it really simple, we just say:

Every 10 levels you gain access to a new tier.
There are a total of 12 tiers of magic.

Lvl 1-10 = 1st Tier
Lvl 11-20 = 2nd Tier
Lvl 21-30 = 3rd Tier
Lvl 31-40 = 4th Tier
Lvl 41-50 = 5th Tier
Lvl 51-60 = 6th Tier
Lvl 61-70 = 7th Tier
Lvl 71-80 = 8th Tier
Lvl 81-90 = 9th Tier
Lvl 91-100 = 10th Tier
Lvl 101-110 = 11th Tier
Lvl 111-120 = 12 Tier

And use the various spells and stuff from D&D as a reference. Like, something on par with Magic Missile being 1st tier, while Lightning Bolt is 3rd Tier and Finger of Death is 7th~8th Tier.
That way, people can easily check themselves and see if their spells are appropriate for their current level.