Morning Dave-

It's a function of the inverse square law. Here's the math, as we know you love numbers Sorry, couldn't seem to get a clearer thumbnail. ( Note- edited to include better thumbnail)
Chart looks bout right to me. Don't forget to allow for reflection- inverse square is only a clean calculation if there's no reflective light. If you were to build the space up and then sample w/ a light meter, you'd find that the math holds good until the plane where reflective intersects direct, @ which point they reinforce. You'll find smaller than predicted loss of intensity as you move lower in the space- still there, but the loss ceases to be purely porportional and becomes somewhat fractional, as the level of reflective has an inverse relationship with the level of direct.

Intensity i @ distance r=