I see it as trying to keep the glass at the half-filled mark, period, because that's where i feel good. Sometimes my head feels cold and that means too much prozac has built up in my system, so I will take a little less for a day or two. I think that's how your doc means for you to manage your meds -not to let the glass go completely empty. Just like a green plant - don't over-water it, don't underwater it. All plants are different though.