Some things work much better in online PC game environment (such as spell points vs spell cast per day) and I think percentage based chances like this are one of those things. Balancing this would be complicated, though, but there's potential.
I'd like to ask devs to keep in mind that some of us really appreciate the variety of builds D&D rules allow... and the transparency of the combat system. In D&D you always know what feat/ability/item/spell does *exactly*, so you can really plan. Please keep this trait of D&D! You're doing a pretty good job so far, but with some exceptions (especially on item special effects and a couple of feats)...