Scooby hi, I think current wind and then a linear decrease in wind strength down to end of descent (thats for FMC planning if you have left the forcast wind boxes blank.
I think the only real place for good gen on the FMC is from the book produced by Capt Bill Bulfer.
about $40. <a href="http://www.firstnethou.com." target="_blank">www.firstnethou.com.</a>
All the best