Lumens - Calculation by Distance

Hi,

First post. Love the site. Anyone able to give or direct me somewhere on calculations what the lumen's a light source will output to a plant based on the distance with given distance and lumens? Ive seen this on wikipedia but it seems a little complex, or I got ADD. Can anyone simplify? An example calculation and the formula would be great.

Spliffrollin

Here ya go!!!!