Examples of Dead Spaces Using One Lamp
These illustrations show how the canopy perimeter shape and resulting square footage can differ in a single-lamp dead space depending on the light fixture used.

About the Illustrations
Illustrations are overhead views for some of the ways grow spaces can be configured, and how the canopy space perimeter (outlined in red) would be determined for a YOR contribution.  The green shapes are matured flowering plant growth.  It should be noted that plant containers too can be static or movable depending on the grower's situation, but for explanatory reasons plant placement is not considered in the illustrations.  A white background indicates space receiving reflected light, a light-managed bordered space.  A black background indicates space receiving no reflected light, dead space.