The room geometry is accounted for in cavity ratios or CR.
The equation for Cavity Ratio is:
CR equals 5 times Height x the sum of length plus width divided by the result of length times width.
Let's calculate the CR for a room with the dimensions of Height=6, Length=12, and Width=10.
12x10 equals120. So CR = 5x6x120 or 660 divided by 120. The result is a Cavity Ratio of 5.5.
For a given space, calculate the CR and then use the fixture manufacturer's CU table to select the appropriate CU.