Upload & Sell: On
Some additional thoughts for you about your 9' seamless.... How big are your dancers, and what kinds of action shots do you anticipate? If you think they're going to be doing horizontal leaps (grand jetes), you're going to struggle with keeping them on the seamless without extending feet off the edges. That's a nightmare to try to fix. Same with height. It's easy to have arms, hands, and fingers go over the top of the backdrop, especially if you shoot to close to her, or from a position that's too low.
Forgive me if this is too elementary, but it's really important, and it's all about geometry. Consider a pyramid figure lying on its side, with the apex being the camera position, and the four corners of the pyramid's base being the extent of your 9' seamless - top right, top left, and floor. Unless you want a Photoshop nightmare, you need to fill that pyramid base with white.
Then, anticipate where the dancer will be in that pyramid, especially as she is closer to the camera position. The closer she is to the apex (the shooting position) the smaller the shooting frame becomes. That's why you should shoot as far back as you can with a lens that can fill the frame with white. Accordingly, larger girls need to be closer to the backdrop (but watch out for your backdrop lights spilling on her), while smaller girls can come forward a bit.
When I set up for a shoot like this, I lock down my camera position so the backdrop completely fills the frame. That gives me the maximum room that my dancer can move in. Anything more than that, and she'd be off the backdrop and out of the frame. So I can shoot this all day long, and know when a jump or leap isn't going to work.
Here's an example. This was shot on a 24-foot-wide muslin backdrop (Westcott Pearl, in this case), which I'll use for large groups. I could have gotten by with 12-foot-wide for this shot, but 9 foot is going to be a bit tight.
Hope this helps.