How do you know when the kyphosis is pathological? 40 degrees? 50 degrees? or above 70 degrees? considering that 30 degree is normal.

Normal is more than: 30 degrees in terms of kyphosis in thoracic spine but many can tolerate greater than 50 degrees with 70 being a number to consider surgery if symptoms and clinical picture warrant but one has to consider other factors like the alignment of the hips and pelvis as well as the lumbar or lower back sway or lordosis as well as any scoliosis and overall flexibility of the spine. See a spine surgeon.