Some potential ways to test include: 1) purchase reference solution for most accurate results likely; 2) compare to test of freshly made up saltwater; 3) compare to water coming out of gfo reactor if you have one, with fresh gfo; 4) dilute tank water with fresh saltwater to see if you get appropriate reduction in value; and 5) compare to results with another kit to see if they're consistent.
Thankfully, I haven't had the experience of a ton of phosphate leaching from live rock but, I vaguely understand that a bunch of phosphate can be 'stored' in live rock so, seeing only a temporary reduction in phosphate post water change, is probably not that unusual (of course, adding food to the tank also increases phosphate).
Matt
