I've had FoK for about a few days and was wondering what the formula was, and I think I've got it so far.
I'll explain. FoK is not like the sheep's electroshock, meaning is does not have as wide a range as electroshock. Say for example you're standing on a map. A monster is a cm. or two away from you, which we will call monster A, and past that monster, there are 5 more monsters in a vertical line down, which are monsters B. FoK does what it's name implies. It fans out. Therefore, If you hit Monster A, then 5 more shurikens will fan out and hit Monsters B. Keep in mind that they must be close though, I'm fairly sure the range that it fans out is as far as the FoK's is. So therefore always hit the closest monster, and keep the mobs close also. This is not a for sure thing, just an observation, so far this works for me. Further experimentation from other players would be great for determining whether this theory is correct or not