help with Hanna phosphate checker

[Amadod2]

so i have a hanna checker and i get different readings with the same test sample each time, now i know there is a time limit on how long the test sample will be accurate for, but im doing all the readings all within 6-9min of adding the additive. when i do my first reading i get .17 and the next two i get .10, im just wondering if any of you use a hanna checker and what is the most accurate way of using it