Wouldn't suggest using furmark, 3dm11 and bf4 are pretty good for testing core. Stay away from Unigine stuff if you don't want to mess around a lot, many nvidia cards have weird issues with them, specifically with boost clocks and offsets you can set. It can be very confusing for new overclockers.
The offset that you set doesn't really matter, only the actual core clock under load. EVGA Precision should tell you that.
It should probably be at around 1275-1325, bit more variance i'm not sure on silicon lottery. If you've got that, you have good OC.
Nvidia clocked 770 higher at stock. Much of the OC headroom from the 680 is already in the "stock" component - especially with gpu boost 2.0 being quite aggressive sometimes. My card clocks 1254 out of the box, my max OC is 1280 - if i give it the 1.212v instead of 1.2v in evga precision, i can get 1293.
Oh and also, if you hit power limit, it won't present itself as instability in overclock. The clocks that your card is using will just drop - same with temperature limit. Your boost and boost offset are "suggestions", not commands, technically.