I think you will be closer to 35-50mm if the model is 7-9feet from the camera (for full body shots), especially using the camera listed in your sig.
I shoot between 5 and 11 feet when doing a model shoot and usually have my 28-70 mounted. I only switch to my 16-35 when I want some funky artistic flavor to the images. And then I am about 2-4 feet from model.
Knowing from experience, the most likely focal length for shooting is going to be around 50mm when going for half and full body shots. I can do a full body with my 85mm from about 15feet (almost out of studio room - actually I am out the room and in to the entryway, but you get the idea ).
My two lenses are the 28-70/2.8L and the 85mm/f1.2L (MkI) for the majority of my studio work. I tend to shoot at f5.6 with the 28-70 and f2.0 with the 85.
I use the 85mm for half body and head shots and the 28-70 for everything else.
Also, unless you have to for some reason, I would recommend opening up the aperture a bit to blur the background, you should be causing the focus to go to the model not the backdrop.
I keep the model 2-6 feet from the backdrop. I am aiming for the 4ft mark usually - it is where my lights are set for, but the model moves around in different poses so 2-6 foot works if I set my lights correctly.
And now for some questions,
- Do your lights have modifiers (such as umbrellas, softboxes, etc.) or are they bare bulb?
- How much distance from backdrop do you have presuming 7ft from camera to model, how much from model to backdrop?
- How high is the ceiling?
- What color are the walls?
(If white or flat black, these are good - if colored more care must be taken).
- Do you have a Reflector(s)?
- What color is the floor, or is it a continuation of the white backdrop?
I or others can help you more with the questions answered.