Unfortunately, we are just beginning to accumulate the data needed to answer that question well. Tom has posted that the approximate ranges for low, medium and high light for many plants are: Low - 50 , Medium - 75-150 , High - above 150. The effect of the light level depends a lot on whether you have good CO2 or not. With good CO2 low light intensity still gives good growth for most plants, but without CO2, many of those plants may not grow at all. I think what we are doing with the PAR meter can be considered to be just gathering information for now. But, comparing your readings to those above will give you some idea about how intense your lighting is, and whether you might want to lower it or increase it.
Also, don't forget that the intensity near the top of the tank will be a lot higher than at the bottom of the tank. If you are trying to grow HC, for example, it is the intensity at the substrate that is of most interest, but for taller plants, it is the intensity in the middle area of the tank. Once you get two readings over the same spot on the substrate, and measure the distance from the light for each reading, you can predict pretty accurately what the intensity is at other distances from the light.
You can also see how uniform your light intensity is from side to side, and from front to back. Ideally, the intensities will all be about the same, where ever you are in the tank. But, you can be sure they won't be.
If you decide to try for more or for less intensity, having two readings over the same spot on the substrate, at known distances from the light, allows you to accuratedly determine how much higher or lower to locate the light for the intensity you want.
