I have had people say when you 1/2 of the water back to change it, some people say when you a full amount back change it. I change mine every couple of weeks, and only add water in the mean time. I have a meter and watch the pH and ppm closely, when the water is being used the ppm stays the same or changes very little you have the mix about right. I always start with a weaker mix than the bottle calls for.
My pH seems to go up every day almost a full point.
As far as using distilled water you may have more problems doing that. Tap water has calcium and other minerals in it and will help your plants. You will have to watch the pH very closely, my tap water has a pH of 7 and I have to adjust it down daily.
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