Question

Why is the reference angle the angle
the terminal side and the x-axis instead of the y-axis? ​

Answer 1

If necessary, first "unwind" the angle: Keep subtracting 360 from it until it is lies between 0 and 360°. (For negative angles add 360 instead).

Sketch the angle to see which quadrant it is in.

Depending on the quadrant, find the reference angle: