Eh, the thing about throwing daggers is, you essentially have a 50/50 chance to hit with either side of a dagger. That said, a knife designed for throwing may somewhat alleviate this by placing weight in a different position, but for western knives it still remains a bit awkward because they usually have/had a handle for throwing, so there's still a chance of hitting them with the handle, which hurts but won't kill.. As far as the range is concerned, 50m is stretching it but it [b]is[/b] possible to penetrate a skull with extremely heavy daggers at that distance, although those require more strength to throw. 50 meters [b]is[/b] pretty much the maximum range on any throwing knife and I think even expert knife throwers would have difficulty managing to do this.

Throwing knives was, besides that, historically never really a thing in the medieval ages because they hadn't figured out how to shift the balance etc, and everyone walking around in atleast a leather jerkin would be pretty well protected against it anyway. Most people simply couldn't aim a knife that well as say an assassin trained to do so. 

I can't say the 50m is realistic, but then again an assassin trained from 4 years old is hardly realistic either, but this remains a fantasy RP so anything is possible right?