There are some spells that directly rolls criticals, for example firewall, but, is this correct?, it doesn't use the caster skill and the defender abilities.
I see more correct to use a ball table as attack, the problem is that any ball table is modified for the base spell, so cold ball table is lesser damage than fire ball table because fireball is 10'R and cold ball is 20'R. I can be wrong, and maybe this is in this way because fire is more destructive than cold.
But, using an attack table you are using the defender elemental resistance, its QU (if you pass quicker you receive less damage) and magical defense (as ball tables don't use any other DB). Too, you are using caster skill as you add spell list ranks to OB.
So, you only create modifiers for any critical result, for example, firewall 'A' critical has a +0 modifier, we could add a +20 for any critical more, so 'B' critical is +20 OB, 'C' critical is +40 OB, etc. or any other modifier (you need to play to try the best balanced one).
As I say, the problem is that you need to search for other elements ball tables, as any element have their good and bad points against any armor type.