Well, if you haven't figured it out already, you can only use offsets with the LE (no way to input a manual Vcore). Anyway I have to use a +.025 offset to be totally stable at 4.5 (my current every day OC. Been tinkering for a while and I'm happy there, for now). I had to use a +.040 offset to get 4.6 to be completely stable. I haven't tried 4.7 yet because I just don't need to, but I imagine it would take at least a +.050 offset, and maybe more, for 4.7 to be totally stable for me.
You may or may not have to use the same offsets, since every chip is different as far as what voltage is required to get to a certain clock speed, so just try it and see what the lowest you can go to be stable is.
Just fyi to give you something to compare to, my 2500k on the p8z68 pro/gen3 is email@example.com stable. Most people I see with the same clock have around 1.30-1.34v
I didn't know that the z68-v le did not allow manual vcore, that is unfortunate.
If I where you, I would start out by manually clocking up to 4.0 ghz with out changing any voltage settings. Do a stress test and see if you are stable with out using any offset. If you find the settings unstable, just add 1 incriment on the offset vcore and test again.
A. If you end up getting to 4.0 with out vcore adjustment, try 4.1, 4.2, 4.3, 4.4 untill you finally hit one that is not stable at auto vcore. Once you hit the unstable clock, go back into UEFI and knock the offset up by one incriment, and then test again. Keep doing this untill you get stable on prime95. Try not to raise the offset by more then 1 incriment at a time as you want the perfect matching settings with out giving it more voltage the needed.
B. If you can not get 4.0 with out adjusting the offset, I would recommend you lower your multiplier down to 3.9 or 3.8 and figure out which one (3.9 hopefully) is stable at auto vcore. 3.7 is the default turbo level so you shouldn't have to go below that at all.
Now while you are doing all of this, the key things to look out for are
It's a popular belief (and Intels purposed maximum safe voltage) that anything below 1.52v is safe. I will personally never go above 1.4v but that is solely because it is my own preference. You need to find out what you expect in terms of clock speed and try to find the right voltage for you that is with in your comfort zone.
It is also popular belief (and also intels purposed maximum safe temperature) that 72 degrees celcius is the limit with temps. I have personally shot my i5 up to 98 degrees before I had my cooler and I did not notice any throttling but I did stop it very soon there after. Recently though, my cpu stays below 70's under load and I like it that way. In reality, during IBT stress testing, having it go into the low 80's won't hurt your cpu.