Who says to start with 300 ppm, and are you using the same nutes they are? I go by what the bottle says. 300 +18 = 318 not 480 but, it should probably be plus your water ppm. Depending on how much water you are using and how many plants that water is feeding, ppm can fluctuate. My water can fluctuate up to 200-300 ppm depending on water level and how long it has been sence I've changed it. I'm feeding 10 plants with 5 gallons in ebb-n-flow.