Originally Posted by MattKing
Did you read the instructions that go with the gray card?
I ask, because very few seem to read them.
The angle you hold the card is important - if done properly, it will tend to give a reading that is about 1/2 stop less than reading the card "straight on".
I knew you had to hold it at angle (1/3 towards camera axis) but this gives around 1/4 to 1/3 stop difference for me. That counts for a bit but still around 2/3 stop out.