Shin Okada
Explorer
Ah and there's my point ... you COULDNT have a +25 bc of the cap.
Chain Dispel's cap is +25, not +20.
It seems an odd question, but how many 3rd level slots (1d20 +10 max) does your very knowledgeable Mage expand before deciding random chance is working against him and perhaps he needs to devote a little more thought into undoing this particular enchantment and start expending his 6th level spell slots (1d20 +20 max) since he's unaware of the difference per-se (in character vs game mechanic knowledge)
The answer is, a very knowledgeable mage who can cast better version of dispelling spell never dare to use the 3rd-level Dispel Magic from the beginning. Especially in a combat.
Sorcerers and Beguilers have plentiful of spell slots to burn. So they just use their best dispelling spell. In case of a Wizard, he should create and carry scrolls of his best dispelling spells at his caster level, or use a staff with better dispel spells.
Seriously, at higher level campaign, lower level spell slots are rarely used, or only used for certain spells which does not have save and caster level cap (say,True Casting, Haste and such). You have enough higher level slots and/or magic items to fight the day. And you should not expect that your lower level spell slots are very effective in combat.
By the way, if you really want to know the strength of the magic you are trying to dispel, Spellcraft check or Arcane Sight and similar spells will at least reveal what the spell is, thus, the minimum caster level expected. I mean, when you see the opponent is under the effect of Shapechange, which is 9th-level spell, you should expect that the caster level is at least 17th. Though there always be exceptions. Arcane Sight also reveals relative strength of the magic aura.