I'm probably missing something really basic, but....how can the armrest drop formula not take into account proportionate upper arm length? It seems to me proportionate armlength can vary quite a bit; as this varies the hip angle is going to vary, right (and how close to parallel your back is to the ground, etc)? Isn't there a relationship between D (saddle height), L (upper arm length) and C (armrest drop)? But the only variable in the formula is saddle height.

It seems that there must be an assumption about proportionate upper arm length to saddle height, which doesn't seem like a safe assumption to me.

I don't get it. What am I missing?