Does/can anyone ever use something as an approximate "grey card"? I'm asking for situations such as if youre out and about and you want to be able to just walk down the street, be at an event, a gig, sports or whatever. Basically a situation where you don't have time to route in a bag for the card or don't have your bag at all. Or you have constantly changing light situations or constantly changing subjects. If you have your camera ready and you want to be able to catch somehting that wont be there 10 seconds later, does anyone use their hand or a grey stone wall etc? Does anyone have any suggestions?