I do the thumb on top for power, index finger for accuracy. ...Just point your finger where you want the fly to go
The problem with my grip is that I sometimes squeeze too tight. One of my 8wts is on it's 3rd cork and right now the one on my 10wt is held together with epoxy and medical tape. I have to make a conscious effort to relax my grip. Not always easy when you have other things on your mind.