Don't forget to turn the true (reported) wind into magnetic (compare with r/w) wind when calculating the crosswind values
Oh - have I completely forgotten all about met theory? Are metar winds not magnetic?
PS: I have never really given a damn until now, having only flown in areas where variation is less than +/-2degrees
Edit:
Disregard - I have already found a good answer elsewhere on this informative forum http://www.pprune.org/forums/showthread.php?t=300546