I used their method and at 5 10 w/ 32" inseam came up with 170 or 175mm cranks.
There was a big thread on this lately and the conclusion as I saw it was that unless you are way outside the norm you could ride anywhere from 160s to 180 and feel pretty good with them. The math and physics seem to concluded that there is no mechanical advantage to running longer or shorter cranks but there may be something said for the ergonomic benefit. It does make sense to me that if you are really big or small you should build your whole bike accordingly including crank length but for most people the standard 175mm cranks will be fine.
Other things can be considered. BB height. If it's low shorter cranks will help a bit with hitting stuff. I did the math in the past as I think I came up with 5mm crank length = about 1 tooth on the chain ring. So running a 180 over a 175 is the same as going to a 31 from a 32 ring. (180=31)