IMO, for a leaper a good starting point is body height. If you can maintain proper posture, go for the length of the body + arms outstretched towards the catcher.
The leap is dependent on her strength. So you can't control the distance. As we all know the closer you are to home plate there is an illusion that the ball will come in faster towards the batter, i guess the closer the better as long proper form is not compromised.
Leaping will give strain to your legs I just want to remind on proper conditioning, warm-up and flexibility exercises for your DD.
Thanks for the advice. With her giving it everything she had she was about 3 inches shy of her body height. I think the six heel to toe distance will be just about perfect for her.