MVP's correct- without a very good analysis of your source water, you want to want to build that into your desired EC/ppm.

This is because conductivity is a very gross measurement- it tells you that something is in there, but not what. This can be likened to weighing a box to find out what's in it- you can establish that there are twenty pounds in there, but you don't know if it's books or rocks- only that it's heavy.

You don't know that the EC of your source water is due to inert contaminants that you can ignore, or contaminants that will sum with the nutrients that you are adding ( also, technically, contaminants from the meter's point of view) to cause an overdose condition. Therefore, you must be conservative and build that number into your total.

When you're in a situation where you have really good, up-to-date analysis of your source water- differant story. F'rinstance, if I saw that my source water showed an EC of .2 ( which would be a ppm reading of 140)-

-and analysis showed it to be mostly calcium:
I'd cut out the Cal-Mag and bump in some Sweet ( high Mg) w/ Pure Blend, or drop the micro in GH Flora by a bit, using some epson salts and an iron chelate on the side, or whatever else depending on what nute system I was using to account for the extra Ca and keep everything else in ratio.

-and analysis showed it to be primarily silica
Well, h#ll then, full speed ahead! but I'd know why pH maintenance was a little twitchy.

- and analysis showed it to be ferrous compounds

Well, I'd feel fairly safe using a three-part, dropping micro fractionally and supping w/ Cal-mag. If it was way bad I'd think About A/B ing it through a filter ( acidify- only need a bit before you add nutes, run through filter to catch precipitates, basify, run through filter to catch precipitates, re-acidify to return to useable pH- you can shake a lot of sh!t out this way, w/o using a lot of chemistry if you think it through.)

Anyway, etc- ya see where I'm going.


In newer muni water systems, you can get a fairly accurate report from the supplier. Older stuff- run ya about $15 thru yer local ag extension.

These #'s can change a LOT according to changes in processing agents, local watertable, weather and usage patterns-
Farmers will generally have supplies tested at least every 14 days.