Light pollution from distant cities?

Hi all...

Doing a little research on light pollution and I ran across this equation:

Intensity = 0.01 * Population * distance ^ -2.5

Where intensity is the amount above the background level and distance is
measured in kilometers....

And obviously this equation is only a guide...but at least its a good start....

My question here is.....at what kind of distances does the curvature of the
earth become a great enough factor that it doesnt matter how big(within reason)
the city is?

Or in other words .....sure the city has a population of 10 million...and the
above equation says the light pollution should be significant but in reality
its only a small fraction of what the equation suggest.....

take care

Blll