I think I used to be able to do the math but I don't know.
Now I use the layman's method - I put a tape measure between the BB and the saddle and I rotate forward around the BB axle. As I rotate the tape measure I see that it needs to go up a bit.
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson