Viewing distance
For a calculated 65-inch 16:9 screen, 30° is about 8.81 ft2.69 m and 40° is about 6.49 ft1.98 m.
These are geometric preferences. They do not say which seat will look sharp, and they are not a reason to ignore the cabinet width.
Calculated 16:9 screen, not a viewing recommendation.
30° is a wider room view. 40° is more immersive. Neither is a medical standard.
8.81 ft
That distance gives about 30° across the calculated screen width. It does not say the picture will look sharp or that the seat is comfortable.