Here's my recomendation: If the Dyna grow nutes you have give directions in teaspoons or whatever and not by PPM, then just follow those guidelines and top your resevoir when needed with plain water. Keep abreast of your pH level especially when using organic supplements. I think pH 5.8 is a perfect all around level although a light fluctuation between 5.4 and 6.2 is fine too. Dump and renew your resevoir at seven or eight days. Run an airstone or two in your resevoir to keep it fresher. I'm finding my plants don't require added nutrients at every topping of the resevoir. I just use plain (reverse osmosis) water. Don't forget that if you have a small resevoir, like 5 or 10 gallons, you can have a service drop off 5 gallon containers every so often, like Culligan or somebody. Culligan's water jugs are all reverse osmosis treated water. The ppm is like 002 and pH is always the same.

Does that help?