Rated power consumption and underclocked are mutually exclusive. They rate the power consumption for the worst chip in the batch under full load at full clocks then add a bit to be safe.

Your best bet is to buy a kill-a-watt meter and use that to see how much your systems power consumption decreases when you underclock the card. Power consumption of a circuit as complex of a GPU has far too many factors involved to attempt to figure it out theoretically, your only option is to check experimentally.