wrong. for the ones with higher numbers, like say 21 pi/3, it goes back to 0 after every 2 cycles so at 6, 12, 18 it would be the same as 0pi/3. so 21pi/3 is 180 degrees. as for the values you have to learn those on your own. pi/3 is 60 degrees so you can make a triangle and have the hypotenuse be length 1, then use sohcahtoa to figure out the sin(y value) can cos(x value) of the angle.
not the best way of explaining it but i gave it a shot