The only times I've used a gray card was when shooting indoor and my camera was connected to my computer using Nikon Camera control Pro. The software communicates with the camera and then lets you set the WB while aimed at the gray card under your specific lighting. So I can't really tell you how to do it w/o the software but on the good side, Nikon allows you do download Camera Control Pro for a 30 day trial and you can use that time to set the WB. I realize this is a bit of a circuitous work around but it does work.