That all depends, really with overclocking you could try and shoot for a speed which you think will be stable, but usually I would just recommend manually increasing. Heres how.
Choose a small increment to increase by, and a program to test with. I prefer 5-10mhz and unigine
Raise the core clock by the speed you choose (again I like 5-10mhz), and hit apply.
Step 3 test it out in the program you selected earlier, and watch for artifacts
Step 4 if you see artifacts back down to your last stable clock speed. If you don't repeat steps 2-4
Step 5 once you have found the max for the core, repeat for the memory clock.
Alternate step 6 Now once you have found the max clocks at stock, you can try increasing the voltage slightly, and repeating the process. With raising the voltage though it is best to use a program like furmark to make sure it doesn't get too hot.
Final step Now regardless of if you followed step 6 or not, after you found speeds you think are stable use OCCT and test for errors, after an hour if there are no errors keep the clock speeds, if there are back down on both the memory and core clocks, and repeat this step, until there are no errors.
Note this is probably not the best method for overclocking, it is just the one I use, and it seems to work for me. Why you don't want to just base your overclock off someone else's, is because if you are unlucky that speed could be unstable for you at any voltage, and two you may be lucky and that overclock may actually be only a fraction of what you card is capable of.