Not being a user of Lightroom, I have no idea...sorry! I use DxO Optics Pro for RAW conversions, and there I'd select all the relevant images, click on the WB eyedropper tool, click on the gray card in one of the images, and I'd be done. Hopefully someone who does this in LR will chime in, if not, Adobe has help forums and the answer can probably be found with a search there or on Google...