Percentage Chance to Dispel = (Dispel Spell Base) + (Caster Level) - (Target Spell Caster's Level)
Dispell Bases
Mordenkainens Disjunction: 20
Greater Dispe: 15
Dispel: 10
Lesser Dispel: 5
-There will be natural 20's and natural 1's (it'll actually be a d100 so >95 or <=5)
-Breaches will work the same
-Breaches occur before dispels
-Each spell gets it's own roll
So, some examples that everyone cares about:
Level 40 dispelling level 7 DW book w/ mords:
(20) + 40 - 7 = 53% chance to dispel
Yes, thats very close to the same situation when the level 40 tries to dispel a level 7 book used by a 40 non caster under the old system. Good. The big difference however, is that he will not be immune to a level 30 caster:
Level 30 dispelling level 7 DW book w/ mords:
(20) + 30 - 7 = 43% chance to dispel
Some other Scenarios:
Level 40 mage dispelling level 40 mage w/ mords (precentage same for any equal caster levels):
(20) + 40 - 40 = 20% chance to dispel
Level 10 mage dispelling level 40 mage w/ mords:
(20) + 10 - 40 = -10 (no chance) BUT... 5% chance to dispel (natural 20)