When you are casting your fly, as more line is picked up off the water the casting arc needs to get wider to maintain straight travel of the rod-tip and to maintain a good loop. So when do you stop the rod on the backcast?

The answer is, "as soon as your fly leaves the water".

Regardless of how much line you are picking up, stop the rod when the fly leaves the water. The rod will no longer load at this point because it is no longer pulling the line against the surface tension of the water. Further casting won't really do anything for you.

If you look at the position of the rod when you stop it and move it the same distance past the vertical on the forward cast, you will get a great forward cast out of it too.