VRay Refraction Value for Lamp Shade Material (LWF)

[kwok3d]

I don't know the best way to do this but whenever i try to use Refract, I ran into problem like this. I like to get a in-between result but 26 and 27 makes big different here. Do you have any idea or perhaps a better way to do this?

BTW, i'm in LWF.