The numbers I use as "standards", 20-30 for low, 40-50 for medium, above 50 for high, are all based on PAR at the substrate level. If you wanted to use average PAR thoughout the tank it would be very difficult to measure, and I don't know what the numbers should be. The same is true for PAR at the midpoint of the tank. I use the PAR at the substrate because it is easy to measure, and because virtually all plants start their growth at the substrate. There may be a better way to characterize the light level, but someone else will have to figure that out.