Chris,

IMO watts/sq. ft. is a little better measure of light intensity because of the varying depth of different tanks. Obviously you would still want to use more light in a deep tank. There is no perfect measure, but I have have found that watts/sq. ft. is better. 21 watts/gal. is probably equal to 42 watts in a deeper tank. As you probably know, intensity falls off in a ratio to the square of the distance from the source.

That means that if the light you see at 2 ft. from a bulb is 1/4 what you see at 1 ft. from the same bulb. At 3 ft. its 1/9 as intense.

Here is a pretty good resource:

http://www.fishdomain.com/lighting.shtml