I meant as you have a fixed NB GTL Ref of 0.635x, by adding 40mv you are simulating having a NB GTL Ref of 0.667x.

Try anything from 0.545x to 0.70x, so Vrefs from 0.71v to ~0.9v.

Eg:
0.545x 1.31 = 0.714v
0.635 x 1.31 = 0.832v
0.714v - 0.832v = -118mv

so to simulate choosing 0.545x you'd choose the closest thing to -118mv

I actually found that my P5Q-Deluxe would prefer really low GTL Refs, other boards prefer higher ones, it just depends.