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.
Ok I think i am beginning to understand Zandor's reasoning. However, if two lights are indeed side by side, the lumens emmitted should OVERLAP at some point within the 12 inches.

So if say I have two lights shining, it is true then that not ALL the space will have the added lumen total.......

Makes perfect sense...

However, again there must be some overlapping of the lumen displacement that is calulated.

Also, if say 5 lights are in ONE BOX, and the light is emmited from a single square foot space with the use of reflectors and the lighting is reccessed, then you should be able to add up the lumens.

Yes?