Happy to explain...
imagine you have a 10' x 10' x 8' high room. 100 square foot, correct? Put a 1000 lumen bulb in the ceiling. 10 lumens per square foot, right?
Now, raise the ceiling to 20'. Still 100 square feet, still 10 lumens per square feet. Do you think you still have the same amount of light on the floor?
Now change the 1000 lumen bulb to a 1000 lumen spot light. Still 100 square feet, still 10 lumens per square foot. But it's very different.
Now change to a LED linear tube, still 1000 lumens. Very very different.
Now change the colors of the wall, the ceiling, the floor, add windows.... Put a frosted lens over the light, add reflectors, add louvers, all still 10 lumens per square foot.
People get confused and you have to give them the benefit of the doubt because lighting is not what most people do all day. They read that the definition of foot-candles is lumens per square foot and think that means that you take the amount of lumens from the lights and divide it by the square footage of the space. And that's not what it means...
Sadly, there is no good guideline, especially with LED. It's best to determine about how many foot-candles you'd like to have, and a style of fixture you'd like to use. With that info, a computer can lay them out for you and tell you how many you'll need and how to space them....like in the sticky thread "lighting layout collection". Or you can ask for GJ members who have similarly sized, colored, spaces and listen to what they recommend.
Hope that helps.
CD
. I hear ya there 