Most algae will live long and productive lives in an aquarium with just enough nutrients and light to fill their needs. And, that amount of nutrients, at least, is less than the plants need. So, getting rid of staghorn algae requires physically removing what is in the tank, or killing it in place, using Excel or H2O2, then making sure the plants have an abundance of the nutrients they need, so the algae is never triggered to start growing again.

In my opinion you are going to continue to have algae problems if you rely on test kits to keep just exactly what nutrients the plants need in the water, without any excess. First, the hobby test kits most of us can afford are not very accurate, and must be calibrated often, using water with known amounts of the nutrient being tested for in the water. Second, the exact amount of nutrients needed isn't a constant - it depends on how much plant mass you have, and how fast it is growing, which depends on the light intensity and how much of all of the needed nutrients you have, including carbon dioxide.

I assume you prefer the water testing approach because you enjoy that part of the hobby. If so, that is probably the best method for you. The EI method would be too simple to appeal to you.