Correct custom white balance metering?

[DocFrankenstein]

What is the correct way to hold the white piece of paper when taking a pic for white balance?

Or rather: How does the 300d measures the white point?

Does it take one pixel in the center?
Does it average a small area near the center?
Does it average the whole image?

One of my friends, while doing CWB said that the white paper should cover the whole image...

This got me thinking.

Another question:
What's a good material to use to set custom white balance?

[Jesper]

I guess it's the same on the Digital Rebel as it is on the 10D: as the manual says, the grey or white card has to cover the middle part of the image (the faint circle you see through the viewfinder, which is the same area that is used with partial metering).

[nosquare2003]

Another question:
What's a good material to use to set custom white balance?

- My first choice is 18% grey card
- Second choice is white paper
- Once I had no grey card nor anything white with me. And the preset or auto white balance failed. I then used the lens cap to do the custom white balance and it's better.