excellent answer Herbie. Thanks for taking the time to answer the question so thoroughly; I have a much better understanding of what you mean now. Could I bother you for one more question, maybe 2?

1) if my water is 250 ppm, does this mean I should only change the res once ppm reads 250 again, or somewhere very near? is this a good way for your plants to let you know that it's a good time to change their diapers?

2) if the feeding schedule is designed according to a weekly basis, does this mean, that if a plant in week 4 of flower requiring 1500 ppm drinks, let's say, 2 gallons in a day and the reading the following day is 1(from 1500 the previous day), that it would be okay to feed her just water for the remaining week?

thank you very much, this has been one of the most useful threads i've come across...and judging by the other responses in this thread it looks like welcome information for many others. imo there should really be a sticky explaining this, because the label on the back of nute products is pretty misleading and vague. i think purposefully, so the majority of customers overfeed and therefore over-purchase, but that's a whole other matter...