The EI dosing method, which that thread is about, is based on:
1. An excess of nutrients does no harm, unless you allow the excess to build up to levels far higher than most people ever experience.
2. If you replace half the water in the tank once a week, the highest concentration of any nutrient will be twice the total amount you dose per week.
3. Plant growth rate is determined by the least available nutrient for the plants - Liebig's Law of the Minimum.
If you dose the amounts shown in the tables, following the schedule shown, then replace half the water in the tank once a week, you will never have a shortage of any nutrient, and you will never have such a high concentration of any nutrient as to cause a problem. None of the nutrients will be in short supply, so the plant's growth rate will be determined by the amount of light and the amount of CO2 present in the water. That is the optimum situation.
Nitrate and Phosphate test kits are difficult to use correctly, often give incorrect results, and need to be calibrated before trusting their results. Rather than trust those kits, use the EI dosing method and don't bother with testing.
Excel is an alternative source of carbon for plants, which can be used instead of CO2 for tanks with low or medium light only. It works best if you dose twice the recommended dosage every day, and don't ever dose the "new tank" dosage (which is 5 times the daily dose).