Something doesn't look right.* I tried to emulate your grip on a TeeBird I have laying around, and I couldn't get a strong grip on my index finger. One tip I have is to first get a good pinch between your index and thumb (I like doing Blake's hammer-pound for this to get the right feel), then put the rest of your fingers on the disc in your preferred orientation (I like the Climo/fork grip since I have small hands) as this will pull the disc back into your palm. Lastly, I loosen everything up a bit going into the throw; while I have small hands, they're strong and squeezing too hard locks my wrist preventing it from opening at the end of the throw.
-Use Blake's hammer-pound to determine your finger/thumb orientation for a good pinch. Your optimum pinch should be pretty obvious after a few hammer-pounds; I also "validate" this by doing light hammer-pound tosses straight up in the air focusing on the disc pivot; if I can sling the disc up the air a couple feet and I feel the weight shift, pivot, etc. it's all good.
-Once you find your optimum pinch orientation, load the rest of your fingers on the disc in an orientation you feel helps maximize the pivot. Again, I like the Climo grip as it helps me get my non-pinch fingers out of the way.
-Adjust the firmness of your grip so the disc will pivot freely, your wrist can get maximum spring, and you are still holding on to the disc (of course). (EDIT: read this
instead, I think my wording might lead you in the wrong direction. It should really be a sling, and the way I wrote it might not get you there)
*I'm not saying it's bad, but it doesn't look right