Most people don't understand how most illumination presents itself as either a bubble of light or to say a spray of light and not even as a complete spray of light.
IE That it starts out small fills up the spread of light to some area and demises at
some distance from the source.
The bubble of light, spray or globe of light is affected by the height and by the type and
wattage of light. So you could get the near same illumination from a 250 W pack at 14'
as a 500W pack at 20' (for conversation).
What your customer needs to understand is that they are asking you to determine
what the foot-Candle will be at some elevation.
In most cases your result will be only 1 or 2 foot candles at your knees.
You can go to this site and use it for a lighting layout all be for spacial areas.
Visual
The lower portion of the site you can add and change many various styles and see many aspects of the light and characteristics of the illumination.
Once you select one light type you can click on the lowest bottom right, and it will
show more additional information of that lights illumination.
Enjoy.
