good basic guide.
best thing to do is go by the guide and then try out some different sizes.
i've found that I really am comfortable on 180cm skis. but it depends.
i really liked the M:EX in 175, but hated it in 185 (tossed me around).
i at first dug my Karmas in a 177, but after riding them in firmer conditions this season hanker for the longer length (they ride short due to the twin tips...for example the 185 Karma is the same length as the 181 Blizzard Titan 9, but you need to deduct even a few more cm for the Karma's raised tips, which means it's probablly more like a 177-179 in terms of surface contact).
the basic rule of thumb I was told when first switching from straight to shaped was to drop a minimum of 10cm and a maximum of 15, all depending on the ski.