Based on the logic of the original system, in which ESF required an extra roll to overcome. . .then letting you cast normally if you suceeded, with no mods.
Essentially, the ESF stuff makes doing it more complicated, so either you do it well, or you blow up. . .
So, without ESF, you do a sloppy 23 roll and it still goes off, but either misses, or barely hits.
With ESF, you do it sloppy at 23, you fail.
You roll a 99 or 100 either way, and it's a spectacular success.
it's not a mod to spell casting difficulty, it's a mod to spell casting failure. . .you are taking more risks and cutting corners or pushing your abilities beyond their limits. . .the odds of success do not change, but the odds of failure go up, and the penalties for failure go up at the same time. (ESF mods to the spell failure table are ugly.)
Well, actually, if you accumulate 98 or more ESF on a single spell, without some sort of bonus to balance it out, you now fail on an UM 01-100, so in that instance, the odds of success are 0%.
that make more sense?