I would set all options, including voltage to default to begin with.
Then, simply raise the multiplier for all cores to your desired level say 40 and test.
Monitor with cpu-Z.
You should see your multiplier at 40 and your vcore under 1.4
When you reach your target, back off a notch and implement adaptive voltage and speedstep.
That will reduce the multiplier and vcore when there is little to do.
Windows power options should allow the cpu to run from about 20% to 100%.
As of 12/04/2016
What percent can get an overclock at a somewhat sane 1.40v Vcore.
Thanks a lot for your answer.
Turns out that I was wrong about Intel Turbo Boost, this feature was indeed required in order to overclock the CPU... I'm going to do some testing !