It is really hard to find an angle that can support both geometrical harmony and a human being's comfortable seating position. After spending hours researching the right proportions and geometrical rules, I ended up using mostly 75°, and tried to follow the path of this angle and its supporting parallels.
In the sketches above, calculated ratios seemed just right. But I had to consider the human body's average ratios to make the chair actually comfortable, so I modified the chair: making it slightly slimmer than the original...
