Okay, I want to learn to master white balance so I bought a set of cards from ebay (white, 18 grey, black) they appear to be just like normal credit card size. I have trouble taking a shot of the card to cover the whole frame. Seems impossible.
Question is that do I really to have the entire cards covering 100% my camera view finder? or can I just take the shot as long as the round circle on the middle of my view finder aimed at the color?
Thanks in advance.

Do you need to set white balance for everything or just leave it as auto on most general purpose, daylight on landscape and shade on indoor portrait?