The only time I have used a gray card in my 45+ years behind a lens was when I worked for Olan Mills Inc. Company policy was shooting a gray card first shot of the day, once mid-day and last shot of the day. Also had to shoot one whenever I changed rolls of film. That was for the processor's use not mine, so the color balance was always near perfect as they could get. I tossed out my gray card when I left them. If you need a subject that renders middle gray for B&W, meter off the lawn(middle gray) or your hand and add one stop(unless you're an albino, then add 2 or 3).