Grey cards are great - but I would recommend choosing one and sticking with it. I have several ranging from basic cardboard made by who-knows to plastic, all weather ones made by dunno-that-either... the kicker is - even though they're "supposed" to be calibrated for 18% - I get a 1-1/2 stop difference between the four cards I have taken readings with the same meter in the same lighting (these are not old and faded cards either)