Offset mode will rely on the chips VID to control the voltage. This simply means... if the chip at stock runs a range from say:
0.8v idle to 1.2v load
When you set the offset to +0.100v it will result in this
0.9v idle to 1.3v load
However this can be quite random as the chip requests more voltage and less voltages at different times. Overclocking with offset can be very difficult as finding a happy medium between enough volts at load with enough volts at idle to keep from instability; isn't always easy.
You'd be best off by trying some negatives in the offset to find how the chip is going to react. I noticed not every Sandy Bridge/Ivy Bridge CPU has the same tendencies on the offset. So I'd start by going back to stock and running an offset of -0.050v or so and start learning where it sits at idle and load and get some averages. Then start stepping up the frequency via the multiplier and watching the voltages from there.