Stress entirely fails to scale properly. It's extremely difficult to make a DC 36 caster level check at level 20, and extremely easy to make a DC 4 caster level check at level 1. Though I think the entire concept is extremely ill advised, you could at least normalize the scaling (Constant + 2*Spell Level).

Also consider that if you make the check such that spellcasters fail half their appropriate level spells out of the gate, classes that use a lot of buffing like Clerics are going to care a lot less than classes like Wizards that are doing more casting in combat.