Just noticed the same website says PO4 should be 0.1 to 1 ppm and iron 0.05 to 0.1 ppm. Seems awfully low. I thought PO4 should be more like 3 or 4 ppm and iron 0.5 ppm or higher.
PO4 can range from .1 to 10ppm, depending upon the setup. It can also be used to slow growth without harming the plants (trying to do this in other ways, e.g.; by cutting CO2 can harm plants). In general, I would target between 1-5 ppm and also try to make it close to a 10:1 ratio with NO3:PO4, but prioritizing a minimum PO4 level. If you have GSA problems, push PO4 to 5ppm, or higher. In my own tank, I target 3-4ppm, but it can drift higher before the w/c. All of my PO4 is provided by my fish. I wouldn’t want to go below 1ppm as this could risk running it out without realizing it.
Iron is mostly a function of chelation type and plant needs (some like a lot of iron). Example: I dose .19ppm daily, but Dwarf Sag likes a lot of iron and I use gluconated iron (Seachem Flourish Iron), which is consumed rapidly by plants (within a half-hour). If I used something like DTPA iron, I would use a lower dose.
Note that my PO4 is a maintenance level, not a dosing level, while iron is a dosing level. When you say “iron 0.5 ppm or higher”, does that mean a maintenance level or a dosing level and, if dosing, what is the frequency?