After reading what Creelo Said, I Wanted to do more Tests.
So i parsed a few tests... On Bees outside Bastok
Using Thockha As a Control (D:47) My crits did between 667 and 716
the Supernal Knife did (D:49) Did between 646 and 782... Leading me to believe that....
Creelo is pretty much right, Its about 10% Increase. So its basically a "Increases Critical hit Damage" Knife (Like X's, or Oneiros) that only works 10% of the time.