How tiny is a dip in brightness of 0.00005? Imagine an apartment building with 5x3 foot windows. Say there are 3000 windows.

5 by 3 foot windows * 3000 = 60*36*3000 = 6480000 sq inches

0.00005 * 6480000 = 324 => 5.4 in by 5 foot, so if one window pulls its vertical blind down by 5.4 inches, you get a brightness change of 0.00005

Apartment building with 1000 windows.
60*36*1000 = 216000

0.00005 * 216000 = 108 sq in => 1.8 in by 5 foot, so if one window pulls its vertical blind down by 1.8 inches, you get a brightness change of 0.00005.