How to get minecraft's accurate 'x' and 'y' rotations?

I’m attempting to recreate this concept using an 16 ‘x’ compass rose and 25 ‘y’ axis degree. My first attempt I already had the rotations for N, S, E, and, W as well the NE, NW, SE, and SW. Additionally, I had the ‘y’ axis degrees looking down. But as I research more, I found the cardinal directions were not accurate.

In the wiki, it states that 180.0 is north, -90.0 is east, 0.0 is south, and 90.0 is west. With that being said the appropriate command for testing facing south is @a[ry=0,rym=-0]. However once a player turning left meeting 0.0 the command automatically finds the player once they reached 0.9 instead of 0.0.

The same issue is the same if a player was to look up. An example is when a player were to attempt to face at 75 degree up the command to testfor such thing would be /testfor @a[rxm=-75,rx=-75].

What do I need to know in order to testfor the correct and accurate intgers? Am I doing something wrong on how the order of ry & rx in each command?