In my office, with fluorescent lights (5000K Silvania), sometimes some tungsten, and a little daylight leaking in maybe, by shooting a white piece of paper first to set custom white balance on my P25+ the colors are almost spot on. This works better for me than shooting a card in the scene, but that certainly helps too. I have found that a white card or paper comes a lot closer for me than a gray card (especially 18% gray). I should shoot a GM ColorChecker card, but I seldom do.
I don't own a WhiBal card (I don't know why, other than been lazy to order it), but I think that this would be better for digital. The gray in that card is better for digital I think, but I believe that Guy can better comment to that.
Edit to add: While my office lights are 5000K, most offices use lights in the range of 3500K plus or minus (some as low as 2800K), however I have found the method of shooting a white card (full-frame) first to set WB is quite accurate, though YMMV.