The AMM from the 737 Jurassic (-200) say:
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The angle of flap extension is measured relative to the wing chord plane.
When the flaps are fully retracted, the trailing edge of the aftflap is aligned with the wing chord plane.
When the flaps are extended to 40 units, the angle between the wing chord plane and the midflap is approximately 43 degrees.
Since the aftflap moves relative to the midflap, the aftflap rotates to an angle of approximately 69 degrees from the wing chord plane.
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