Basing it on CL would mean that a CL40 epic wizard will more seriously screw up casting prestidigitation than a lowly apprentice. Seems counter intuitive. If you want it to scale faster, I'd recommend something like 2*spell level, and only have it apply to your 2-3 highest levels of spells. Unless you are playing in a very different system, there is no reason that a high level caster casting mage armor at the beginning of the day should have a random chance of summoning Orcus.