IMO setting setback by KOPS is somewhat random due to varying body proportions - what we're really trying to do is achieve balance between saddle and bars, little weight on the bars. I think KOPS works fairly well with 170mm cranks. That's a first approximation, then we move the saddle fore and aft to adjust for balance. Crank length doesn't really matter at all. What matters is one's fore and aft relationship to the bottom bracket, i.e. where one's foot is at the bottom of the stroke. That determines where one's butt is OOS and the general balance of the bike.
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