The Estimative Index is a method of adding fertilizers in excess of plant needs, then a big weekly water change to reduce that excess.
It is not set in stone, but a place to start.
Here is how I do this:
Figure out how much I will use in 1 week. Put that into 2 bottles:
macros in one bottle, micros in the other.
Figure out how much I want to dose daily. I dose 1ml /3 gallons of tank volume.
I cannot dissolve the ferts any more concentrated than that. So add up all the gallons
of all the tanks, and put in that many milliliters
of water into each bottle. (ie: I dose 1000 gallons worth of tanks, so I put 1000 ml of water in each bottle)
M, W, F dose 1ml/3 gallons of macros.
Tu, Th, Sa dose 1ml/3 gallons of micros.
Su = 50% water change.
Next, I tested the water after 1 week, and found in some tanks (heavily stocked, moderate light) the NO3 was climbing more than I wanted it to, from fish food.
Since fish food has N, P and most traces, I modified the recipe so I was dosing a lot less N, P and traces, but added K2SO4 for potassium, and chelated iron.
Once you figure out how you want to dose you can mix larger batches. You can keep them refrigerated, or add Excel to the bottles. I find the Excel does not actually stop the goo, but slows it down a lot.