I'm using GreenLeafNutrients MegaCrop 1-part. Their website has a feed calculator that reads out in grams, EC, or various other units. It has a note that the feed recommendation must be added to whatever the starting EC (or ppm) of your water is. to quote:Finally learning a little about ec and ppm and I have a possibly silly question….if you have a target of 700ppm and your water is 400 are you adding just 300 in nutes? Or add 700 in nutes to read 1100? I’d think just add 300 but an alchemist I am not.
*EC is additive, so you measure your base water EC then add in the suggested amount.
For example: Base water EC .2, suggested dosage is 1.1, then final level will be 1.1 + .2 = 1.3
Some of the old timers may have advice on your water, I don't know if 400 is high. I guess I'm lucky mine is 140 ppm lake Erie water.