I am fiddling with that problem at the moment. I have been using the EI for a few weeks. I noticed that my wisteria had bright yellow veins, so I tested at the end of the week. My nitrate was OK, probably >15 ppm, hard to tell from the Seachem test, but my phosphate was <0.5 ppm. So currently I am doubling my amount of phosphate dosing (4 mL of Fleet per week instead of 2 mL) while keeping my nitrate dosing the same.
Correct me if I am wrong, but I believe the basic premise of the EI is to not allow nutrient levels to drop below what is needed by the plants. This is what I have come to believe from my reading. I am not really paying strict attention to ratios per se. That said, the EI is only one form of dosing, many of us use bits and pieces of different dosing strategies.
Perhaps my fish load and feeding is providing more nitrates than phosphates. Or maybe my plant selection is consuming more PO4 than NO3. I'm not sure.
As far as extra K dosing, I plan to hold off on it for now, and merely watch for deficiencies. Since my fish load has increased substantially over the past few weeks, I plan on testing my nitrate more frequently, and if I need to dose less KNO3 because of the fish load, I will seriously consider adding some K2SO4.
Wasserpest I am always interested in what the gurus are doing in their tanks. I know Momo was trying the EI for a while.