I use lamotte test kit, had same problem, i dont know why but i cut down on my po4 to once a week about 2 month ago, so far so good, i did not caliprate the kit, i have the water company test results saying 0 po4, which my kit say. am not an expert, i dont pother with callipratin, i just use the ket to tell me if things are very low, ok, or very high, you can calliprate if you need acurrate results, i did caliprate in the past but really did not make much of a diffrent.
if your kit above the sclae you are most likely at the high end.