I would recommend leaving the voltage on auto to start, and just bumping your multiplier up. With it overclocked as such, stress test with prime95, and open up CPU-Z to see what sort of voltage the "auto" setting is giving it to run at 4.2ghz.
Then go in to your bios, set your voltage to fixed mode, start just under the voltage that the auto voltage was giving the processor, and keep bumping it down a bit at a time until it becomes unstable and then you will know the minimum voltage your 3570k will need to run at 4.2ghz.
THEN, once you have that voltage figure, go back into your bios and start tinkering around with offset voltage. You will have to select an offset voltage, and then stress test to see what your voltage is. Then adjust...