Marc please check your facts closer someone may believe you have a clue about what you are doing someday. Try to understand what you are trying to say instead of just copy paste stolen information from another website.
Calculating foot-candles or lux is a more accurate way to estimate the amount of light the plant receives. It does not measure how much light the plant uses.

The correct formula is I=L/D²

Intensity = Light output divided by the distance squared.

The reason you can't add the watts or lumens up is because they don't share the same space. Even if they are next to each other it's still not the same space. Proper light placement is most important. Light intensity diminishes exponentially very fast.

Lumen is just another way to measure the intensity of the light and is a very accepted unit of measurement. I know they should have asked you for your opinion but hey they came up with the formula I would guess before you were born.

Light from 0-12" has the same power in lumens it's after the first 12" that we need to deal with. I will finish up the drawing I am working this weekend and make light placement and it diminishing effects into an FAQ.