Move the decimal to the left 2 spaces. 90% would .90. 74% would be .74. 134% would be 1.34 Got it? It' always the same no matter the percent. 2 spaces to the left.
Sin angle that equals the 1/2 value is the 30 degree on the 1st Quadrant.
The angle of 30 degree on the 3rd Quadrant is equivalent to (formula and circle in the 1st photo) 180 + 30 = 210°.
So theta = 210° (3rd Quadrant) = 30° (1st Quadrant).

Now just transfer the cos 30° to the 3rd Quadrant and the signal will be negative (as shown in the 2nd photo attached).