I basically agree with everything Gavjenks has written above, but the one thing I would do differently is to place the background light not on the ground, but at the mid-back level and aimed straight back at the background. This looks great on half-body shots, where the athlete's body blocks the backlight stand and flash. When a light is shot from low in the center of the background and aimed up, it's brightest at the bottom, and fades darker pretty quickly. When the light is level, and aimed straight back, the halo is more even, and it avoids the really awful potential side effect of a low- or high- angled background light, which is showing off ripples or wrinkles in the background material if it is paper or muslin, or if it's a textured wall, it can reveal the stucco texturing.
If the shooting space is very cramped, and the background light to background distance is SHORT, with the light low, the light is VERY "hot" where it first hits, and then it can easily be 4 or 5 f/stops less in intensity at the top of the background; this makes a bright background very "low" on a person, which looks, well, weird many times. If the background light is LEVEL, the falloff is much less-noticeable, and it just looks...well...more natural and less "lighted" or "lit up".
WHen you have a small-sensor camera, in a small room, you end up using SHORT focal length lenses to get a full-length figure, and at even 12 feet from the person, the depth of field is deep enough at f/8 for the background to become an almost-in-focus, and recognizable element. This is one of the main differences between APS-C and FF and 120 rollfilm sized shots that are made in front of seamless backdrops. If you have an APS-C sized sensor, you might want to try and keep the depth of field shallow-ish by shooting at f/5.6 or even f/4.8 and focusing VERY carefully.
As for the background light and grid: you might, or might not, want to use a grid. It depends on the FOCAL length of the lens, and how much background it "sees" in your camera room. With a grid, you might not have enough beam spread to get any kind of a halo, so you might have to ditch the grid. Where that light is placed depends on the kind of modifier it has, and what effect tyou want to achieve.
A second idea is to light the off-side of the person with the 4' umbrella AND with the SAME umbrella, aim it partly at the background, so that THAT umbrella also lights the background a bit.