Overclocking requires trial and error, there are no "best settings" since one graphics card may not be able to attain the same clocks as another identical card.
When overclocking graphics, things can go bad quickly so you want to use very small steps to increase your GPU speed and graphics memory (my personal rule for graphics - no more than 50MHz each step). Run your benchmarking/stressing tool (
Unigine Heaven is good), after each increase and look for display issues like tearing, artifacting, strobing, etc. Also check to see that your frame rates have changed, if no change in frame rates, there's no point in increased clocks. As soon as the display doesn't perform exactly right, you'll want to back off to a previous setting that wasn't problematic or slightly increase the voltage (monitor temps if increasing voltage, you should be doing that anyway). It does take time and trial to get your clocks dialed in properly - under no circumstances should you put the settings at their limits to begin with since that's likely to overwhelm the graphics
If things should go wrong and you lose your display, starting in safe mode should give you the ability to remove your graphics overclocking utility (and overclocks with it) and start over