For what it's worth -
There's a two hand geometry involved with this. My left forefinger, at the first joint, hooks around the trigger guard. My left thumb lies on top of the slide release. My right thumb locks the base of my left thumb. I am not a pro or a competition shooter, but it is a natural "grip", and for me really locks the sight window and the recoil window.
But then again I've got pretty big hands.
P-01, by the way.