How to set up GTL Ref Values for 45nm & 65nm

[Bobly]

My Vdimm (memory voltage right?) is set to manufacturer specs which are 1.9V 1600MHz, 7-7-7-20, it should be stable at those settings. On a smaller overclock I had the Vdimm set to 1.86V while still remaining stable, I only put it back up to 1.9V to stay within specs.
What's the rule of thumb/a good voltage to start with for nb gtl? Should I then raise it higher or lower? Is there a maximum one shouldn't go over?

S2E w/	Q9550		REF0	REF1	REF2	REF3	POST	BOOT	IBT	STABLE
FSB	1866		Auto	Auto	Auto	Auto	X	X	X	FREEZE
Multi	8.5		-29	13	13	-29	X	X	X	Testing
Memory	1600		-21	13	13	-21	X	X	X	FREEZE
CPU-Z V	1.36V		-21	26	26	-21	X	X	X	FREEZE
PLL	Auto		-14	26	26	-14	X	X	X	FREEZE
VTT	1.28V		-7	26	26	-7	X	X	BSOD	
Memory	1.9V		14	39	39	14	X	X	FAIL	
NB	1.5V		28	65	65	28	X	FAIL		
SB	1.5V		42	0	0	42	X	FAIL

I'm noticing that negative GTL References seem to fail less than positive ones, could thing mean something else? Maybe my VTT is too high?