Hi Horace,
I am the guy that Ricky is quoting. I also chimed in on page one of this thread.
While I am certainly not in the 0.05% area that jdieck is in, I am hopefully in the 10% area.
My method is pretty simple, I have used several phosphate removing products and three different test kits (Merck, Hanna colorometer, Salifert). I have used all three tests together several times at several different phosphate levels (ranging from 0.01 to 0.08). I stopped using Salifert after a little while because it was too difficult for me to read. I did the other two tests together approximately 10-15 times over the range of readings and found that they correlated pretty well.
While I have a good undergraduate Chemistry background, I have in no way shape or form tried to understand the chemistry behind the test kits to the level that I would have any useful opinion as to the accuracy of any of the tests (including Hanna).
What makes Merck better then Salifert (the only other test kit I have used recently) is that it is much, much, much easier to read then the Salifert. The Salifert may very well be completely accurate but it is too hard for me to read.
As far as the comparisons between the phosphate removers, the only one that was able to lower my phosphates below 0.03 was the PHOSaR HC. It has been consistently at 0.01 since I started using it and hasn't changed yet. That may not be a completely accurate number due to test kit and/or operator error, but it seems to me that the trend is there.
The other thing about the PHOSaR that I like is that the phosphate numbers dropped and stayed there. With the other phosphate removers 0.03 was the lowest it would reach, but the phosphate levels would bounce around between 0.03 and 0.06.