Originally Posted by Maris
The problem comes from the fact that grey cards are not perfect Lambertian reflectors. The"greyness" varies with the angle of the card versus the angle of the light versus the angle of the meter.
The instructions with the card say to point it midway between the primary light source and the lens axis and meter from the camera position.
Amazing how easy things are when you RTFM (Read The Fine Manual).
“Wise men talk because they have something to say; fools, because they have to say something.” - Plato