50D custom white balance zooming

Yes, I meant zooming in on the picture when having chosen "Custom WB" in the menu, then pressing the "SET" button when done. It seems odd you can zoom in/move around if it's not possible to select a section of the photo.

My gray card is credit card sized, so pretty small when using a 50mm lens or similar meaning I have to move very close to get a shot of almost only the gray card alone.