I see using the gray card in a reference shot as much more than an approximation. In fact it calibrates the print to the scene. It factors out exposure errors and even differences in film or development choices.
Let's say I've done my paper testing and I can program my EM-10 or color analyzer to reproduce middle gray (the Kodak card tone) reliably on whatever given paper is in the enlarger, and I set the enlarger properly. I should get really, really close to a "real world" match every time.
My argument does assume real rather than relative placement and that shadows and highlights are simply allowed to fall off the paper where they may in a straight print.