Reference Measurements
reference light intenstity = 100 fc / 1076.39 lux
reference subject distance = 10 m

Target Subject Measurements
target subject distance = 20 m
intensity at target subject distance = 25 fc / 269.10 lux
exposure compensation = -2 stops

To match Target Subject Distance's exposure to Reference Intensity & Distance
intensity needed to match reference intensity at target distance = 400 fc / 4305.56 lux

* sanity check note - Different lighting technologies, modifiers, diffusion, beam angle, and emmisive source shape will likely have an impact on light fall-off over distance. Not to mention other variables like bulb age, COB variation, and other gremlins that may be toying with your light output. This tool is designed to give you an approximate values to help plan your lighting setups. As always, test as much as you can prior to the day. When in doubt, overprovision.