How do you determine offset?

[Twirlywoos]

I've overclocked my i53570K to 4.6Ghz and it's cooled by a Corsair H60. But how do I determine my offset value, so when in 2D clocks (idle desktop) the CPU operates at a lower clock speed/voltage?

Thanks.

Also if I want to push for higher clock speeds (5Ghz) should I upgrade my cooler to a H100i?