Welcome Ben!
The overhead technique you and Leon (and many others) use is good for short-range accuracy. I recommend reading Bill Blohm's “Slinging advice for beginners” in the articles section. Lots of people email me saying that there was little tidbits of advice they found helpful.
It’s pretty obvious that longer slings are trickier because the extra length causes the pouch to rotate faster (and provide additional power). This in turn makes it harder to hit the right spot + or - a few degrees, because the ideal window of time is compressed. There's really no way to avoid this unless your rotate slower, which would defeat the purpose.
In order to take real advantage of extra cord length, you really need to become a slinging master, which is no easy task. And really, to be a true master, you need to have been slinging from birth. Slings are still a part of growing up in several cultures in Africa, many pacific islands, and others. If you've been exposed to slings from birth, it becomes second nature; a true extension of your own arm. It's like throwing a ball, something most of us do from very young. If you first started throwing a ball when you were 25, you wouldn't be nearly as proficient, even with years of practice. But to most of us, it's a piece of cake. Now, that’s not to say you can’t become very skilled. You still can with practice and patience.
It’s all about consistency with slinging. Forget about hitting a tin can for now. The key is to miss in the same place every time
. Once you’ve gotten the consistency, then it’s just a matter of tweaking your release so that you shift your misses onto the target. If you are firing all over the place, don’t invest the time in getting it on the target every time, it just won’t happen and you’ll get frustrated. It’s your release that’s letting you down. Focus on getting a consistent release and then getting pinpoint accuracy is attainable.
Keep up posted on how you do. And as always, we’d love to see some pictures of those slings!
Chris