Deny requirements are directly tied to success (+4) results though. If they roll 6 dice to cast and get 3 (+4) results to deny you have to nullify those 3 sucesses. If they got 4 sucesses you would need to deny 4 times. Ect. Ect. Deny is deny all successes or the power goes off.
The better question here is if you declare the full power at temp but only get a lesser success is that a failed cast or just a cast at lower power.