You really need to invest in a set of test kits and test your water parameters regularly. pH, ammonia nitrite, nitrate, Ca, Alk, Mg and Phosphate for starters. If you decide to dose anything else, get a test kit first and check the level before you dose. I prefer the Salifert test kits for Ca, Alk, Mg, Nitrate because the colore change is dramatic and I can easily get a true reading. I have a hard time with the test kits that have gradual shades on the color charts. I also use a refractometer (aprox $35-$50) to check specific gravity (SG) and I use a meter for pH now.
Are you mixing your salt 24 hours before doing a PWC?
You can get a RODI unit on eBay for around $100. Another great investment.