When i first got my P45 board a few months ago, I knew nothing about GTL Refs, but I wanted to know how to set them. I found there was scattered, sparse and often conflicting information on how to set them up. I derived the 0.63x for 45nm from reading everything I could and noting what people were having most success with, and for the most part I kept seeing people saying that 0.65x - 0.70x was less stable for them than ~0.63x (this was mainly people with 45nm quads) and people with 65nm quads were able to clock better on ~0.667x. I experienced the same thing with my 45nm dual, if I set 0.667x it became unstable, the best setting was ~0.635x when using 1.2 Vtt for me.

It's just a starting point, and I made the guide to consolidate the information I'd read and my own findings together to give people a basic idea of what to set to begin with. I also explained how to set up the GTL Refs using the formula Asus use in some of their bioses as it seemed to be confusing some people. Thankfully it has turned into a healthy discussion with many people contributing.

Essentially GTL's need to be adjusted when you're trying to stabilize your cpu at higher (400 - 450+) FSB and higher clock speeds, under those levels it will most likely have a negligible effect. You start with say 0.635x if you have 45nm, and 0.667x if you have a 65nm, and tweak from there based on your FSB, clock, and Vtt, guided by the individual response of your cpu to the adjustments you make. Even if you have a 45nm you may end up at ~0.667x giving you the best results, or you may have a 65nm and end up at ~0.635x, there's no hard and fast rules to it. Generally the higher Vtt you set, the lower your GTL's need to be to compensate.