I recently switched to a claw grip after experimenting with different mouse grips and that one felt the most comfortable to me.
As for the question, I usually hold my mouse harder with my thumb, ring finger and pinky (not too hard though, just enough so I feel like I can comfortably move it around) than I do with my index - and middle finger. I do this to get more control with the mouse. I also tend to curl up my pinky a bit to get an even steadier grip. After I feel comfortable holding the mouse with my thumb, ring finger and pinky I just place my index - and middle finger on the mouse 1 and - 2 buttons in an "arch"/curl (or whatever you like to call it). I might do some small adjustments to get a comfortable and repeatable grip every time, but that's how I usually hold my mouse.