Personally, I think that it should be removed on the other end (as I think that the caster should ALWAYS have a chance of fumbling - or else you are playing D&D). Have the target have a static Resistance (much like DB), and then the attacker has to roll over that number.
For example, a RR could consist of the following:
50 + racial mod + stat mod + defender's level expressed as a "rank bonus"
Note: "level expressed as a rank bonus" means +5 for each of the first 10 levels, +5 per level for the next 10, and so forth.
Thus a first level human caster, with a racial mod of 10 vs Essence and a 80 in each of his Intuition/Empathy/Presence would have Resistance Bonuses of:
CRB = 50 + 0 + 5 +5 = 60
ERB = 50 + 0 + 5 +5 = 60
MRB = 50 + 0 + 5 +5 = 60
A first level wood elf would have the following:
CRB = 50 + -5 + 5 +5 = 57
ERB = 50 + -5 + 10 +5 = 62
MRB = 50 + -5 + 10 +5 = 62
And a first level dwarf would have:
CRB = 50 + 0 + 5 +5 = 62
ERB = 50 + 40 + -5 +5 = 92
MRB = 50 + 40 + -5 +5 = 92
So, for a first level caster to cast a first level Sleep spell on these three, the caster would have to roll above the ERB (Essence Resistance Bonus) of the target.
The Casting Bonus for that caster would consist of +1 for every rank in the spell list, the realm stat bonus, and their level bonus. Thus a first level human Magician with a 90 for his Empathy and who knows the list to 5th level would have a Casting Bonus of 5 + 10 + 5 = 20.