Tom, are you saying that the test should be done approx one hour after dosing to get the most accurate reading?
More like the test methods used, either error on your part or the test kit is off by 5X.
I'd say the latter.
1 tsp to my 180 should add 5ppm extra PO4.
I dose this amount 2-3x a week.
Run a test against the known standard for the test kit if you wish, see the sticky at the start of this subform for more info.