I'm not shooting the messengers,
but that Palm stuff is bunch of hooey that he pulled out of his ar'se to suit his own preferences. I've read his site a zillion times, and it's based on nothing more than his opinion.
21% puts me on 188mm cranks, which is so crazy that I can't even get my head around it; and 21.6% would have me on 193mm, and I'm only 6ft!!!!! Insane!
About 5 or 6 years ago i used 180s for about 12 to 15 months (I even bought 3 pairs of DA 7800s), but gave them up as a bad joke, and now use mostly 172.5s. I could never get "on top of" the pedal at the start of the downstroke with the 180s, plus I was having medial knee pain.
I makes me 'cranky' that Palm says that knee pain from long cranks is a myth or an old wive's tale. Sure, most people go to longer cranks with no problems, but plenty do
have issues, including me.
Below is Kirby Palm. If this fat 'expert' came up to you at a bike shop and told you that you should use his crank length formula, because it "works for him", would you listen?
Does he look like an expert who's based his formula of years and years of riding many, many mles?