Can somebody help me with entity directions?

I wanted to create some daggers that deal additional damage when used behind the target (similarly to the mod Backstab for 1.12.2).

I thought that if the player is attacking the target from behind, it means they have a similar direction. So I made a condition that checks if the absolute (if it's negative, turn it to positive) difference between the direction of the player and the target is littler than a set value (I chose 50).

However, after a few playtesting, I remembered that the direction is relative to south and can vary from 0 to 360. I tried working around that, but I later realised that the direction system is somehow weird (west, for example, sometimes it is considered as +270 and sometimes as - 90...).

After placing a circle of armorstands all facing a different direction and using a procedure block that sends a text showing both the player's and the target's direction, I managed to almost fix it by turning anything negative into positive and to have a condition that checks if a direction is higher than 180 and, if it is, to set it to 360 minus that value.

It works almost always, the only issue now is it doesn't really distinguish west from east so it triggers the "backstab" effect even if the player and the target are facing each other, if the directions are east and west.

So, after spending a whole afternoon trying to find a solution I began to hate armorstands... Does anybody have an idea? XD

(If you need it I can post the procedure here)