Any paper that only reflects 85% Lambertian isn't white, it's light grey. Do the experiment guys. Photograph a double thickness of reasonable quality copier paper and compare the histogram in the same light with white chalk, virgin snow, fluffy white clouds or any other surface that's recognised to have close to 100% Lambertian reflectance. I've also heard it said that the optical brighteners added to copier paper result in a reflectance > 100%. You can't have it both ways. Whatever, for all practical purposes, if you take a reflective meter reading from white paper and open up by 2.5 stops you'll arrive at exactly the same exposure as you'd get from metering an 18% card.