I am by no means an expert on this, but a number of years ago I spent some time playing with throwing knives to see if I could figure them out. I had no formal training, just wanted to be up front about that.
I started with a basic grip of the blade, and arbitrarily picked three different points on the blade at which I would grip it: the point, about half way up the blade, and more fully up the blade. I then practiced throwing from each of these grips, to try and get used to how quickly the knife would rotate when thrown from each grip. I started with just a half rotation (gripping the point, the knife only has to rotate half way around to stick in the target). Of course this was a very short range throw, but was easy to figure out the distance for each of the grips. I also tried to be as consistent as possible with how I throw each time, not over flick the rotation, but rather get a nice, easy, and consistent rotation each time.
Then, I started to work on a one and one half rotation, for a longer throw, again working it from each of the three grips and figuring out the distance. From there, one could go on to two and one half, three and one half, etc. rotations for greater distances, but it definitely became much less consistent in sticking the target.
My reasoning was that as I became proficient in judging distance I could choose an appropriate grip and number of rotations. For something that seemed in between, I could pick the most apropriate grip and then "fudge" the throw a bit with some extra flick to rotate the blade a bit faster, for instance.
My approach gave some mixed results, I never became terribly consistent at the greater distances, but perhaps the approach has some merit.
Another thing I did was to practice juggling the knives. This gives you more experience in timing the rotation of the knives so you always catch them by the handle.
