I believe the "departure" is from the predicted high water tide, which is based upon the sun and moon's position relative to the earth. With Bill in the neighborhood, the departure from mhw is expected to be approximately a foot higher than it otherwise would. So if you look at a tide chart, which doesn't take into account atmospheric events, you can add a foot. Bear in mind that tides always change either a little or a lot from one cycle to the next even in the absence of massive storms because the positions of celestial worlds are always changing. So what I'm saying is that you're essentially right....
Have fun tomorrow.