Distance changes exposure by a ratio.
For an ideal point source, illumination is proportional to 1/distance². Double the source distance and the illumination becomes one quarter: a two-stop loss. Increase distance by √2, approximately 1.414, and the illumination halves: one stop. The important quantity is the ratio of the distances, not the number of feet you moved.
Moving from 2 feet to 4 feet is a two-stop change. Moving from 10 feet to 12 feet is about 0.53 stop. The same two-foot movement has a different effect because it is a different fraction of the original distance.
Use falloff to choose the background relationship.
Suppose a subject is 4 feet from a point source and a background lies another 4 feet directly behind them along the same light path. The source-to-background distance is 8 feet, so the background receives one quarter of the illumination at the subject. That is a two-stop difference in incident light—not a promise about how bright two different surfaces will look in the photograph.
If you move the source so it is 8 feet from the subject, that same background is now 12 feet away. The difference becomes approximately 1.17 stops. Moving the source back makes the illumination more even across that depth, but also reduces subject illumination at unchanged power. The placement planner shows the trade-off and the output needed to recover the subject exposure.
The model has conditions.
A large softbox close to a subject is an extended source. Beam shape, feathering, grids, surface angle and room reflections can also change what arrives at each point. Do not use a single inverse-square result as a guarantee of uniform skin exposure. Measure at the points that matter, with comparable meter orientation.
Use the measured falloff tool to compare actual readings at known distances. A curve fitted to those readings describes that tested arrangement over that tested range; it is not a universal specification for the modifier.
YOUR NEXT 10 MINUTES
Predict. Measure. Refine.
If your fixed-output flash meters f/8 at one position, predict the reading at twice the source distance.
Keep the same beam direction and meter orientation. Meter at the original and doubled distances.
The point-source prediction is f/4: two stops lower. Note any departure rather than forcing the result to match.
Repeat with a large modifier close to the meter. Save both configurations and label the modifier, distance and diffusion.