Yes, the angle the card was held made a difference in the reading and could be a source of further errors.
Originally Posted by Stephen Benskin
I should clarify...
My 18% gray card in the Darkroom Dataguide 1970 (measured 0.78 reflective density)
But since averaging meters provide the correct exposure for 90% of all pictures, the purpose of the gray card is to be what the averaging meters are aiming for. They aren't aiming for an average gray that is in the middle of the characteristic curve. They are aiming for a brightness distribution that the average photographer gets when he points his camera at the average picture. Which usually includes a bunch of sky.