I know I should just try this myself but can anyone explain the difference in effect achieved between a large light source (softbox or umbrella) close to, verus far from the subject, say a head and shoulders shot.

Theoretically I know this: Light fall off would be much more apparent with the light closer. With the light farther away, the background and the subject would be more equally illuminated (less pronounced light falloff).

Closer light means a larger light source, farther light means smaller, harsher light.

Does anyone have a rule of thumb, something which works for them to determine how close to place the lightsource, other than, I need it close to get f11?