The luminance value is from the profiling software. The associated brightness level is not so direct. Wayne Fox gave me a simple app that tells me the setting. It goes something like this:
tell application "System Preferences"
set current pane to pane "com.apple.preference.displays"
tell application "System Events" to tell process "System Preferences"
tell slider 1 of group 2 of tab group 1 of window 1 to set x to value
display dialog x
That tells you the brightness level the mbp is set at. Mine is 0.6875. That setting corresponds to a luminance value that is my target. So if I am on a plane and want to change it temporarily (or if I hit the brightness keys accidentally), I can change it back and check that the setting is indeed 0.6875.