They are all adjusted rolls - with the 5% fumble being a constant.
Assume that the casters each have 6 ranks and +10 for stats for +40 overall in their spells.
The caster casting an Elemental Attack spell has a 5% chance of fumble, otherwise their spell works - though it may not hit the target, but it can still deliver collateral damage.
The caster casting an Attack spell has a 5% chance of fumble, otherwise their spell works - though it may not get through the target's defense and is unlikely to deliver any damage if resisted.
The caster casting a Utility spell has a 30% chance of failure and 5% chance of fumble, otherwise their spell works.
Elemental Attack spells have it best. Why not use the attack roll as a casting roll? If the adjusted spellcasting roll fails to exceed 70, then it fails. If it exceeds 70, then it becomes the attack roll or sets the RR target.... If 70 is too high, then lower it to 10 and treat it similar to Attack Spells. I personally like the way I have always done it, which yields a chance for a Double, Double x2 or Triple result for a great spell casting roll.