Yes, this has  been discussed here before.  Many of us feel that we can achieve greater accuracy with minimal loss of power and range by slinging at eighty five or ninety percent of our full strength. 
   One other thing that slinging has in common with martial arts... if you're doing it correctly a lot of your power comes from your hips. 

Thanks. It's good to have someone to discuss this with.

One thing I have learned from martial arts is how to aim my training at a goal: Moving slowly and tying the movement to my breathing and heart rate allows me the time to encode precision into my movements and observe the feeling of that precision. Should I need to execute the movement fast, the urgency of the situation will naturally increase the speed of my breathing and heart rate, and thus the speed of the movement.

When holding static postures, joint pain indicates poor alignment and makes the necessary corrections obvious. Muscle fatigue compels the body to compensate to find the most efficient version of the posture, using only the effort necessary to hold the posture and relaxing everything else. This is allows subtle movements of the core muscles to be transmitted in whip-like fashion all the way to the fingers and toes.

Since I am interested in the  sling as a hunting weapon my priorities include accuracy, minimal preparatory movement and making that first shot count.

To train towards these goals, Here's what I do: I try to hit my target with the first shot. If I miss, I take a long break then another "first" shot. When I get a hit, I celebrate and congratulate myself on hitting it the "first" time (fake it till you make it) and that is the end of my training for the day. I might continue to throw, but that's playing, not training. The number of "first" shots it took to hit the target is my number for the day. The goal is for every day to be a 1 and that means I am hitting my target 100% of the time during training. If I am having an off day, I get more training while on days when I am on I get to play more, so it's win/win.

Training this way got me rather quickly to long strings of days that are 1's and it is rare for me to have a day that is more than a 5 or 6.