Quote Originally Posted by herbie the love bud
So the general rule is you should have a Cal/Mag mag ppm of 300 per gallon. So with tap water or with reverse os, pour a gallon and add your Cal/Mag product to bring the ppm to that level and make a note of how much you added (mine is 3 to 4 mL per gallon).

Now everytime you mix your nutes you know how much to add without measuring.
What kind of water are you adjusting? My nutrients are formulated for water with an EC of 300, which is 210 ppm; I have to aim for that I guess.