Find out what is in your tapwater first by getting a technical water report, not the consumer confidence report that is typically provided. I use the Aquarium Pharmaceuticals pH, KH&GH combo, Nitrate, and Phosphate test kits. I dose to hit the middle of the recommended ranges shown here:
I think in the long run you'll do better dosing the individual ferts rather than an all-in-one mix. Greg Watson sells individual chemicals too.
KNO3 for K+ and NO3, also sold as stump remover
K2SO4 for K+
I use Discus Buffer - monosodium phosphate for PO4
Either Flourish or CSM+B for traces
Baking soda for KH
But you need to know what is in your water to determine if anything needs to be done about Calcium and Magnesium - the components of GH.