Of course you need to shoot the card in the same light as the pictures.
Thanks for the suggestion. However, how well does this work for landscape shots, when the light often varies substantially over the frame? (For example, when some of the frame is in sun but a some of it is shaded by trees, mountains, etc.) That's the only thing that has kept me from trying the gray card technique. Is my concern at all significant, or should I try it anyway?
What I have found to be most effective is the Colorwasher plug in from the Plug In Site
Thanks, Bob. I'll give it a try and see how it works for me.