Every time you buy a new bucket of salt, run a full line test (or bring it up to the store), and check it's buffering capacity. You'll know if/how much buffer you need to add for that bucket.
FWIW, pure RO/DI water will not usually register a true pH, KH or GH value, and salts that are made to be used with RO seem to take that into account and put more buffer in the salt mix.