Repeatability and correctness of measurements of angles above 150 degrees

Why is there no stability and repetition of measuring angles greater than 150 degrees? When measuring them, the length of the tube being measured and the straight sections at the bending point are constantly changing. Is there a detailed instruction for achieving stable operation when measuring configurations with angles of more than 150 degrees? Is it possible to make a threshold for measuring the angle from 120 degrees? Contact for communication +79051832483 whatsapp, viber.