Calculating luminious range

Calculating the Luminous range of a light.
Let’s say the nominal range of light “X”, extracted from the Yachtsmans Almanac is 20 nautical miles, and the existing visibility is 11 nautical miles.
What we want to find out here is the luminous range.
Entering the luminous range diagram, we locate the nominal range of 20 nautical miles on the bottom horizontal line.
From there we draw a vertical line upward until it intersects the curve representing the existing visibility of 11 nautical miles.
From this point we draw a horizontal line to the right, intersecting the vertical luminous range scale at 21 nautical miles.
Therefore the Luminous range of this light “X” is 21 nautical miles.