"I dont understand how people are saying keep the plant at this ppm at this time?" Experience
First of all, I would suggest going to the manufacturers website for your chosen nute brand and read their directions for use. Then use some common sense. I.e, if you use DWC, your plants roots are sitting iun solution permanently, in ebb & flow, they are flooded for a period then drained for a much longer period. SO, it is natural to assume that the strength of nutrient solution with E&F needs to be higher than with DWC.
As a general rule, I would suggest going with HALF of what the naufacturer says, if the plants seem paler green than you would like, up the strength to maybe 60%. If you see broiwning leaf tips, lower the strength.
In my DWC with AN Sensi nutes, on the bottle it states 4ml/l. However, on the website it states 4ml/l max strength for BOTH bottle, giving 2ml/l each bottle. Highly confusing and caused me massive problems till I figured it out. Once I started feeding my plants 1.5ml per l of each bottle, I got fantastic results with something like White Widow (very nute sensitive) but average results with NL#5xHaze. Once I upped the NLxHaze to 2ml/l each, fantastic results. (This is the experience bit)
Each strain is slightly different, some like more, some less, its your job as a grower to work out which is which by reading the signs the plant gives you. Green, upturned leaves=happy plant, yellowing downturned leaves=unhappy plant.
Be patient, learn what the plants tell you and ask loads of questions, especially about stuff you dont understand, someone will always help and if you need, pm me.
Green mojo dude.W