Thank you CryptiK and mikeyakame

But how do I know what the default value for CPU GTL ref are when I calculate, lets say Q6600 on a P45 board and I can set the GTL for all cores.

Are the default (auto) like this on a P45 board

GTL Ref 0 = ,635
GTL Ref 1 = ,667
GTL Ref 2 = ,635
GTL Ref 3 = ,667

And if I want ,667 on all cores I hade to ad (+) some (how much is depended on the VTT value) for 0 and 2 and let the 1 and 3 left at auto so they all are at ,667.