For Me,the use of a gray card with the D70 is a bit redundent as the D70 in auto white balance will give you great results under all but flash lighting situations,my preference is to shoot exclusively in the raw mode and adjust temps in PS,its a work flow less prone to dissapointment,however,one technique for useing a nuetral grey is,while out shooting where color and texture is varied,to place a card or any nuetral object in in the line of sight of the subject your shooting,fill your frame and shoot it,and then later while in PS you can bring up a levels layer,use the nuetral eyedropper,click and than save as a history action,and then apply to a shot taken under the lighting conditions