Yea Exolent, but that way will give false results some time, such as you shot from above, sides, under player, and so...i maked a plugin some time ago to fix backstab bug, where you make ~> 100 dmg with knife.Take a look and feel free to take what you need.
You can adjust the back angle
#define BACKSTAB_ANGLE 45.0 - max angle between target player aim origin and attacker player aim origin.