You can look at PAR as a local measurement at each plant, and at various areas of that plant. Or, you can just use PAR as a general way to determine if you have the light level you desire, measuring it only at the substrate. I like the latter, because it gives me one number I can use to define how "good" that light setup is. It is very hard to try to set the PAR for multiple points in the tank at the same time, so why even try. When we say 25-35 is low light for PAR at the substrate that doesn't mean the whole plant sees that light level at all parts of the tank. Every part above the substrate sees more light than that, but the tank is still a low light tank. Similarly, if the PAR at the substrate is 80, that isn't extremely high light, even though all areas above the substrate see much more than 80 PAR. I hope that isn't confusing.