Test kits need to be used correctly to base your routines and mainteance schedules, period.
You cannot rationalize away they are accurate "enough", for montoring when you simply do not know if they are or not.
If you want to guess, then just look at the tank, see wha other folks do as far as a routine etc.
Why even bother with the test kit at all then?
If you use a test kit, then you will want to calibrate it, this way you can rely on the measurements(and the cost and labor associated with test). Then you can make a decision about what to do for water quality, whether or not do to dose something, do a water change etc.
What good is a guess for monitoring? You need to confirm whether or not that test kit is correct over that particular range.
A test strip that measures say PO4 is not going to tell you whether you have 0.4ppm per day uptake etc out of the water column or not. It might be 0.1ppm, or i might be 1.0ppm, who knows..............so what good is this info? What if you assuem it's 1ppm and when it's really 0.1ppm? How far off and misleading can test kits be? Pretty far in many cases.
If you plan on using them for predictive purposes, dosing etc, then make sure to make a set of say 1ppm 5ppm, and 20ppm ppm's ranges for each nurient in question. I suggets Lamotte and Hach test kits, I have for over a decade now. Colorimeters and lab research also calibrated and uses a reference standard to calibrate the readings prior to running your sample/s.
So even a 20,000 $ lab machine demands this for good accurate results, now someone wants to suggest that their 5 $ strips are accurate enough?
Fine if you are okay with it...........just do not use that data to make any sort of claims for support
You don't get to have that trade off, unless you like lying to yourself about things and assuming things you really have not checked out.
That's is why it's confirmed in the first place with a reference standard.
This way we know.
I know that this is a bit more than many, if not most hobbyists are willing to chew off. I know many will not spend the $ for the nicer test kits and then not do the calibrations, even if the how to make them is spoon fed to folks, they simply are just not going to do that step. Then many simply do not test consistently.
So rather than fighting that, I went the other way with EI, and avoid the test issue altogether. Many howled and some still about the method and how bad it is since oh my, it says not to use test kits.
It's ironic though, those claiming the utility and usefulness of test kits often rally against calibrations and high grade test kits. They want dirt cheap stuff, not to do the work, think someone else's calibration applies to them and their test kit without doing it themselves(very common) and also claim it's "accurate enough".
What the heck? And EI is inaccurate? haha
You do not get to use the cheapo stuff, cut labor, methods etc, and still get the same results. That's the trade off there. You can attempt to rationalize it to yourself, and say this does not matter to you, but then you slip down that slope that says being accurate does not matter anyway. You dilute your very argument that suggest you need testing in the first place.
Going to other way, you can be confident in the test kit measures using reference solutions over the range of interest. Higher grade testing equipment is easier and nicer, easier to read results, better resolution etc.
Then you know and that data can be used to make a decision about dosing, water changes etc.
If you want to argue "need", many tanks do not ever need testing of any sort.
That was hersey years ago, still is too many today, but it's rational using something like EI, large water changes on reef tanks etc, works well and avoids that issue. Folks like to avoid water changes also, so they look to test kits.