I would absolutely say FIRST thing your horse needs is to stop straight and square. No leaning, no gawking, stop straight, hind end underneath her.
Next, understand that when doing rollbacks in cutting, it isn't a simple rollback. When facing left, your horse should pivot on her back LEFT foot. The outside rear. This will set up her body for the next movement and put her in the correct lead, as opposed to having to transfer weight to the left after the turn, it saves a few extra steps, and helps you better keep up with a calf.
Getting these down first is more important than putting a horse on a calf or dummy.
Make sure that you get the bend mentioned above, as you need to be able to steer your horse with her ribs instead of her shoulder. So if you're coming along at the calf going right, don't use your right foot to push her closer to the calf, use your inside, or left foot, to bend her ribs around the calf instead, as it will encourage her front end to move toward the calf. Driving with the ribs is imperative because it encourages them to lift the inside shoulder and move closer to the calf, but with a cutting movement, more of a jump around to face or turn the calf as opposed to crossing over in front and peddling around. If you would like, I'll scroll through some videos on youtube to find what i'm talking about. If I can't find one, I'll make you one tomorrow at the barn.
edit: a good way to practice the bend and know if you've got it is to walk forward, with your horse in a banana-shape. If you were looking at her from above, she is in the form of a banana, and when walking "forward" she will in essence be coming diagonally across the way.