I have watched your video and would like to give you some tips.
1. Try lifting the sling over your head (this is scary at first, but you'll get comfortable with it.)
2. dont lift your leg, push your hip towards the direction you want the stone to go.
3. keep your sling parallel to the ground.
4. a 45° angle will give you the optimal range.
5. learn to put a rifle spin on your ammo.
If you follow these tips (and i dont suck at teaching
) you can easily get ~100 meters