Caveats to include:
1. Not every spell should hit every monster. i.e. you need to pick a spell that attacks the mobs weak save.
2. Spell Pen is an additional defense that some mobs may have, but we can discuss that elsewhere, please.
3. Lets be honest, this is mostly about enchant, conjuration and/or necromancy. (Yes, conjuration too. Web is, i think, the most commonly used spell to attack a reflex save if you are not a dps caster. Then again, I learn new stuff all the time.)
To make things simple and have a place of reference, how about we use 50%? (This does not include debuffs.) Just what DC should hit the average mob on epic elite (end of the road/GH end game stuff) on a roll "to save" of 10 or lower?
My initial stab at this would be 50.
A few people would have the really awesome DCs of 55 and get a 75% chance and not have to do as much debuffing as a result of the extra effort they put into that school. Those with lower, say even a 45, could have useable results after applying a couple debuffs.
Again, the question is, what DC should hit a mob's weak save 50% (before debuffs)?