The 18% gray card is so abused and misused, I feel bad for it....The Kodak instructions that come with the card explain that you have to position the card at 1/3 of the angle between the camera and the main light both horizontally and vertically (by angling it, less light hits the card so that it is not at 18 percent reflectance anymore). Then you open up by 1/2 a stop over that.
If you like olden days film lore, you were supposed to include a few frames of a gray card in all of your shots as a neutral reference for the color timer. Kodak also suggested using a gray card to tell your timer exactly what you had in mind for exposure. If you wanted your film to be printed darker, you were suppose to expose your film as normal, but include a shot of the gray card overexposed, for printing lighter you underexpose the card. That way, when the film was printed, the timer would use the image of the card to set the exposure of the print and your image would look just the way you wanted it (theoretically).