The guessing for landfall is very late Friday night or pre-dawn Sat AM. So the tides may be inbetween (but it is only worth a foot + anyway, and the surge is over 20 feet, so it doesn't matter much). But speed of the storm seems to be one of the most unknown variables in any event. One just doesn't know.
Ordinarily the timespan given begins when tropical storm force winds are forecasted to reach the coast (i.e., Friday evening for Rita on the TX coast). The surge will be at its worse when the eyewall passes over the coast. Tides will make a major difference, even if the surge is 20 ft+. You don't want high tides. The astronomical tide is added directly to the hurricane surge. If the tide is 4-5 feet, then add 4-5 ft whether the storm surge is 3 feet or 30 feet..