Yo !
"[...] Each damaging spell cast has a 15% chance to activate Arcane Missiles. Chance doubled for Arcane Blast. [...]"
That's what the spell's description says. I think the general 15% proc chance is well scripted, but the 30% proc chance on Arcane Blast can't be good, that's impossible.
I get procs there and there on different spells, but Arcane Blast is for sure the spell which gives me the fewer Arcane Missiles procs overall. I feel it's more like a 5-10% proc chance max. There's no way to test it precisely, since it's RNG, but can you guys verify this ? I spammed Arcane Blast on a dummy, and I got like 1 proc out of 15-20 casts when I'm lucky, absolutely not the described (almost) 1 out of 3 ratio.