Hello all, I seriously need some pros here to enlighten me about this thing known as the 18% Grey Card. I have search all over the place for a 18% grey card and I can't seem to find any. However, CP is selling the Spyder Cube and the ExpoDisc slapped with a pretty high price tag. How do you actually use the cube? Do we measure the WB using the "PRE" WB function for the cube? Hope my question seems clear. thanks