You can use a protractor.
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right angle!
You find the center of a screwed 45 degree elbow by bisecting the angle.
It is easy to draw an equilateral triangle without a protractor. That gives a 60 degree angle. It is then simply a matter of bisecting the 60 degree angle, using an unmarked rule and compass, to get a 30 degree angle.
60 degrees
By using trigonometry