From page 24 of the X38 datasheet.

Note: Unless otherwise noted, the voltage level for all signals
in this interface is tied to the termination voltage of the Host Bus (VTT).
As it pertains to the chipset in this intance VTT is taken from the same source that supplies the processor.


From page 284 of the same datasheet.




Notice inside the red boxes? 0.666xVTT_FSB. Northbridge GTLREF is set from VTT.

When it's all said and done all that really matters is one comes up with settings that work.