Get in a safe area where the stone will not hit anything important.
Then load your stone and just swing it around without releasing it.
This will give you a feel for keeping even tension on the cords. Usually it is "slack" in your cords due to bad technique.
I sling stones larger than tne pouch all the time.
Centrifical force will keep it secure.
It can only come out if it is not centered in the pouch---more weight is outside than inside the pouch or if there is slack that allows the stone to move out because centrifical force has been lost.
Hope that helps. Then again, a split pouch presents a different story. I was assumming a solid pouch