As it happened, I went from yoyoing when walking the dogs to slinging.
Staff twirling before yoyos and juggling before that

An hour a day where my hands weren't doing anything, always seemed a shame to waste that kind of dedicated practice time - so I didn't

These days it's mainly thinking time. Pretty much always got something percolating through the gray matter.
And the combination of exercise to get your blood flowing and increased oxygen intake definitely gets your brain working at a higher level.
So yes, sliding loop on my index finger was just second nature when I decided to try slinging

35 plus years agoit also turned out to be the final dog walking skill.
And I'm still learning

The rest of the grip evolved from there.
So your daughter's definitely on the right track

If you get blisters on your retention finger, you're gripping too hard.
Another advantage of the three knot system, you have a very secure grip, but with minimum applied pressure.
The hardest to use release I've ever used was one of jaegoor's long slings, it had a flat wooden toggle. You had to grip that extremely tightly and with decent sized ammo, it was always in danger of slipping and sending the rock off at some random angle.
Watching back my wheelchair video, I was struck by Just how delicate my grip looks.
Never really noticed before.
Almost like someone drinking tea with their little finger sticking out lol
But even with rocks weighing several pounds, it's never felt anything but totally secure.