Got it working. Thank you for pointing me in the right direction!
Show Menu: Classes
Other adjustment subcategory: class
Eval Script:
Phase: Post Attributes
Priority: 20001
Code:
~ If we're not enabled, get out now
doneif (field[pIsOn].value = 0)
~ If nothing chosen, get out now
doneif (field[pChosen].ischosen <> 1)
~ Adjust the max caster level up or down
field[pChosen].chosen.field[cMaxSpLev].value += field[pAdjust].value