We are talking about the effectively used part of that large screen, and the wattage that lands on that. Looking around the border of that effective area, about half of that large diffusion screen is not being used.
Here's a quick drawing that has approximately the proportions of Matt's simulated set/setup. Lamp on the left, "girls" face on the right. You can see the parts of the diffusion screen that are not being effectively used. As a notion, throw them away. Or, better, leave all alone, change nothing, except swap out the source lamp for something about 1/4 the watts, filling the approxamately matted effective diffusion area.
Hopefully that's clear. I'm hoping that Matt, who I think probably understands the physics well, or is at least able to simulate it, will drop back into his own thread.
Edited by Gregg MacPherson, 09 December 2015 - 11:16 PM.