Well basically offset mode is a trial and error process. The first thing i would do is manually adjust your voltage during your stress testing when trying to achieve 4.5ghz. Once you have manually adjusted your voltage to a stable voltage for you OC then you want to change to offset. What i did was I went into my bios changed my Vcore voltage from Manual to Offset than saved and reset. Once system reset i reloaded bios and looked at what the vcore is set at as default with no +/- added to the offset. From this point you should +/- the difference between the default offset to what your voltage was before. I found this guide on Overclocking with Offset voltage : http://forum.overclock3d.net/showthread.php?t=38867.
I havnt read it but it may help you with understanding offset voltages. An expert on this forums might be able to chime in a little more or tell you what i am saying is garbage but this the way i did it.
Note: take into account the Vdroop. Depending on what setting you have for your CPU LLC. At one point my droop dropped voltages from 1.16 to 1.132 witch than BSOD my machine. After playing with the CPU LLC Levels i got it at 1.152-1.168 during stress tests.