Is there a good substitute for a gray card?
Yes, there are several. Hold the palm of your hand in the same light as that of your subject and incline your palm at approximately 45 degrees to the light. Meter your palm with a reflected light meter and increase the suggested exposure conditions by one stop if your palm is pale and a half stop or not at all if your palm is dark. This produces exposure conditions very similar if not identical to those from a gray card. Calibrate your camera bag. For example, the sand colored canvas of a Domke bag returns approximately the same reading as a gray card. See http://idiom.com/~elight/earthlight/photo_tech_notes/color_zone_p1.html for other suggestions.
