The arm/hand and leg/foot moves from the shoulder and hip much like a whip or the dial on a clock. In other words, the closer to the point of rotation (center of circle...) the smaller the movement. The farther away from the point of rotation, the greater the movement or arc. So, rotating the buttocks 20 degrees means the thigh should move 20 degrees or more. Bending forward from the hip (with IK on) the hip might bend 2degrees, the waist 4, and the chest 6, to simulate actual body movement. This is a rule of thumb, tho not carved in stone.