It's mod + level, so prime doesn't change your roll. A prime has a lower base for success, so you need an 18 + spell level to cast successfully. No matter how you look at it, the stumpy elf is just as smart as the barbarian girl... but the 2000 year age difference must mean something on Rolz because you nailed magic in one try, like the karate kid with the fly in the chopsticks! (Worst. Simile. Ever.)
Mythic Scribe
Valar Lord
Shadow Lord