I suppose you could try to estimate how much moisture is in the acetone, and try to add a stoichiometric amount of anhydrous salt. But to tell the truth, I've never known anyone who's gone to that much trouble. Once, during organic chem class, I added a small amount of anhydrous salt to dry my product, and instead of just sitting there, absorbing the moisture, like every other time, it crystallized into a mass of clear Epsom salt crystals, eating up a chuck of my product. And all my teacher had to say about it was "Oh, you should have distilled first." Yeah, well, thanks ... for telling me now. SO you see, sometimes, people just aren't as exact as you'd expect they'd be, given the specificity of the rest of the procedure. It just happens that way.