not a particularly heavy rock, I was slinging much heavier ones on the beach in mallorca.
Best technique for heavy rocks is a simple straight arm rotational underarm.
So longest sling depends on how tall you are. standing up with arm hanging down, sling must be just off the ground.
Mine would be about 33 inches.
You could easily get a 2lb rock 100 metres.
I was hitting a small cave about 50 metres away with them quite easily.
for the record that's about the weight of rock Pep Salinas uses for target slinging
He is a big lad
I can figure 8 a rock that size - but wouldn't recommend it for anyone else.