Overclocking using offset voltage?

[clownshoes]

Currently I have my 2700k running 4.2Ghz @ 1.20v. I've been messing around with the offset because I don't want it running at 1.20v idle (just wasting power). When I switch over from manual to offset (with offset on auto) the load voltage is around 1.30v with idle at 1.00v. If I set the offset to -.1 my BIOS reds it out implying that I shouldn't use that setting. Am I using offset incorrectly? How am I suppose to get the processor to downvolt at idle when my computer sets such a large discrepancy between idle and load voltage by default? Any help is appreciated.

Thanks