I'm just starting out in this planted tank aquarium world (about a month or so old) and it appears to me that there are two methods of calculating how much light should be introduced into your tank. The two methods being, Watts/Gallon and PAR. The Watts/Gallon seems way oversimplified for our purposes and the PAR method seems a bit daunting in complexity. Again, I'm a newbie to all this (although sometimes a new set of eyes can prove useful) and hopefully what I'll put forward here is something useful, not too complicated, and not oversimplified. This is NOT intended to take the place of Watts/Gallon or PAR. The sole intent here is to determine how many watts of light are actually reaching your plants, not how much you need.
The Beer-Lambert Law relates the absorption of light to the properties of the materials through which the light travels. The calculation is very straight forward and can be quickly written in MS Excel. The Beer-Lambert Law is written for each media type the light travels through (for our purposes once for air and once for water and then combined), see the equations written below. I think I've also used a pretty typical small plant aquarium setup as an example.
Again, what the Beer-Lambert Law tells you is what is the effective amount of light getting to your plants based on the color of the bulb, the distance of the light bulb from the water surface, and the distance the plants are beneath the water surface. It does NOT tell you how much light you need.