I am familiar with the usage of a (Kodak) Gray Scale card when photographing, e.g. to get a color-proof replica of a painting. Now, I'd like to use such a card to ensure my scans are color-proof. At photo school, we slightly unfocused the card to achieve a homogenous color area.
What do you suggest is the best method to adjust that for scanned images in Photoshop. I'd tend to use the blur/average, but I wonder what's the best selection size to apply that effect.