exposure - How do you use a gray card?

There are already two questions on how to make a gray card, but unfortunately my ignorance goes deeper than that.

How do you use a gray card?

Answer

There are a few ways. Here are the main two I use:

1. 
   

   before you start shooting a scene/pose have someone hold the card in your primary light and hit it with your in camera meter. (in spot meetering mode, ideally with the card fully filling your spot meter area, even if that means zooming in.) set your exposure such that that is "properly exposed". Now remove the card and shoot away. Your main light source should be properly exposing subjects.

   
   


2. 
   

   first shot of a session (or anytime you change the lighting substantially) slip the grey card into the shot in the main key light. Take a "throw away" shot with the card in it. If you have multiple light sources with radically different color temperatures, you'll want to shoot one with the grey card illuminated by each light source. When you're editing, use the grey cards in those shots to get your white balance adjustments, then mass apply them to the entire shoot.

   
   

There is also the "historical" use with dark room printing... shoot a frame with the card in it, even if it's way off to the side out of crop, then in the enlarger, slide the film so that you can print that bit, do a test strip of the card, develop, match the box on the test strip to your grey card to get the initial printing time for the negative.

/me wanders off to FIND my old dektol stained gray card... been using a modern white/grey/black target with digital lately... not sure where it is. ;)