Hmm interesting. Yes the color chart helps a lot.
How much is their photometer?
I found a couple of digital photometer tests that were pretty nicely priced a while ago. I wonder how they stack up vs. the test you have? I'm not sure we can get that iron test kit you have in America.
Digital test kits: https://www.aquaticplantcentral.com/f...tml#post664222