Sorry, I was too vague. You multiply by 100, I just thought it was pretty much a given. I believe this is actual attenuation, it makes sense, you are calculating how much the beer, actually, attenuated.
DB is correct - the formula is for apparent attenuation. That is what we measure with our hydrometer. As alcohol is less dense then water and our fermenting/ed beer has alcohol in it it will throw off your hydrometer reading. While of theoretical importance actual attenuation has little practical use in homebrewing as all attenuation numbers we use are of the apparent variety.