When the aircraft flies at 850 mb, the standard reduction to sea level is 80% of the flight-level winds, from 700 mb it's 90%. The reason is due to the typical wind profile in a hurricane, with a peak in winds at about 500 m above the surface, with a sharp drop off below that due to friction. If it weren't for friction, the strongest winds in a hurricane would actually be AT the surface. As such, the 850 mb winds are actually stronger than the 700 mb winds usually, requiring a larger reduction to get the sfc winds.
Thanks. Now if I can remember that at sunrise, that will be a miracle.