To calibrate your test kits you need to test them against a range of reference solutions. This ensures that 1. Your test kits are accurate for a range of values and not only accurate at one value or off on all values and 2. That if your test kits are off you can adjust the results based on know variances to ensure that you are dealing with good numbers. Even the good test kits can't be 100% trusted to be accurate unless you check them. They're just more likely to be accurate or close to accurate. Since you are dosing to specific values, then you need to make sure that you are dealing with good numbers.
If you have a lot of fish that you are feeding heavily and your plants aren't growing fast enough to keep up, then your Nitrates and Phosphates with rise as a result. The issue is to determine whether you need to feed your fish less, determine why your plants aren't keeping up, or accept that your fish to plant ratio is such that you don't need to dose Nitrates and/or Phosphates.
Now if you're short on another nutrient like Potassium, then it will be harder for the plants to use up the other nutrients like Nitrates and Phosphates. If the plants can't use them as efficiently then quite often you'll start to see the nutrients which are in sufficient supply start to rise. Remedying the deficiency will allow the plants to start using the other nutrients and stop or reverse the climbing levels.
Hope that helps.