Just find test's where both GTX 295 and the new 285 contains. Your overclocked 280 performs like then 285 so, just check how far its from the 285 up to the 295.
Here's a good example, a great and reliable test with both cards:
It's in swedish i know, but that' doesn't make much sense. Just pick the game u wanna compare and u'll get the results! Cheers...