its really hard to damage a GPU unless your temps are too high, you basically want to raise the power limit to max, whatever your card can handle to mine for example can go up to 139% power limit. then for a start hit the core with 80 Mhz and the mem by 250 Mhz while Heaven Benchmark/Valley Benchmark are running they are free. if everything looks fine no Driver crash/ artifact on screen, then go ahead and add 10 to the core and 50 to the memory. if you hit a limit where your card is unstable. (Example: Crashing/clocks all over the place) try and add a little bit of voltage like 10mV i generally like to stay under 40mV make sure to keep an eye on Temperatures.
Of course i use MSI Afterburner.
For reference i got my GPU at 1527Mhz core and 1987Mhz Mem but i got lucky with my card
people tend to try and get 1500Mhz on core and 2000Mhz on mem but that is what i managed to get on my card