When light shines into an aquarium the air-water interface slightly focuses the light. Beams off center are refracted towards the center. This is why the PAR around the center of the tank goes up when you add water. Near the glass at the front, back, and ends, there is considerable reflection of light back into the tank from the glass-air interface, which increases the PAR near the glass. But, if the glass is less than perfectly clean on either side of the glass the amount reflected back into the water is decreased. As I recall I did those PAR measurements in a 10 gallon tank, which I didn't take pains to clean thoroughly. The glass must have been non-uniformly dirty. This was a few years ago, so my memory of that isn't perfect.
Thank you very much for the explanation. I understand what you are saying. The air-water interface focuses light somewhat like a magnifying glass does (maybe an odd shaped one) and light reflects off of the sides, front and back somewhat like a mirror. The disparity between the front to back measurements are understandable.
I don't have either a 30 inch long tank, nor a 24 inch long light to do PAR tests with. Most of the data I have comes from others who either post it or PM it to me. Someone with the right tank and light, and access to a PAR meter, needs to do that testing. One thing I do know is that if you raise the light far enough above the top of the tank a short light can light up a tank well enough for our purposes. In fact one good way to light a tank is to use a short, but very bright light (4 bulb T5HO, for example) suspended a foot or more above the tank. That greatly reduces the difference in PAR between the water surface and substrate level, and gives relatively uniform light over the substrate.
I don't have a 30" tank running either, but I read a lot of posts from people that have them. 30" is somewhat of an odd length when looking at lighting options. It seems that lighting is basically designed in one foot or 12" increments, like 24", 36", 48", 60" and 72". This makes 20" and 30" length tanks having fewer lighting choices. 60" fixtures and bulbs for them may be somewhat of an oddball length too.