Skew control

What is a good value for it?
These setting were previously stable (before I update BIOS version and have the option to control skew) 1.23125vcore / 1.175 vtt
Skew - 0ps = unstable (fail at 7 mins)
- 200ps = looks good (passed 30 mins)

In the best case, I don't really want to do again 2-3 hours test, so which value should I take that is "normal"?

Try 900ps

Do you know if it reduces a lot the performance?

Well you would need to use an oscilloscope / logic analyzer to calculate the correct value for CPU Clock Skew, as there is no "normal" setting when taking into account clock jitter and timing resolution among other things. It's more of a trial and error setting, since the 900 ps setting that works best for me may cause instability w/ your setup. But since we're both running EVGA P55 boards, albeit different CPUs, a setting of 900 ps has helped me push my 860 to almost 4.4GHz at 1.35V.

I see but I don't get it why we can't left it on auto then... I mean the computer should itself be able to determine the right value with much more precision!

200ps seems good so far. My first intuition was right I guess
I never achieved 4.4GHz under 1.4v, I'll try 900ps