Your temperatures are still in the safe zone. This is thanks to a better then stock cooler. That said, the max will vary from person to person. I say if its under 70 in prime, you should be good to go as long as it is stable and not running a overly high voltage.
Do not push the base clock too much more as at some point it will start to become unstable.
You may be able to get better results in the bios with your own settings. Most software and auto overclocks use more voltage then needed
Check here, but i personally use no pll over voltage(2600k). Just take it slow and keep an eye on it. stress test it. go higher if you want. At some point, you will not see or feel the difference, that is when i would stop(or if temps start to move up quick or you need large jumps in voltage). In most cases you will just need a bit of cpu voltage(if even) if you are just going in the 4.4-4.5 rangehttp://www.tomshardware.com/forum/265056-29-2600k-2500k-overclocking-guide