Titration based test kits that are offered in the aquarium hobby are virtually useless when measuring ph0sphates. It has nothing to do with the quality of the test kit or the manufacturer. Instead, it has everything to do with the limited resolution of the test kit and how low a concentration of phosphate is usually present. The best way to measure phosphate concentrations in your reef is electronically with tools like the Hanna Low Range Phosphate Calorimeter or the Hanna PO4 Checker. They measure down to the 1/100th ppm phosphate with an accuracy of +/- 4%.