In that case, I think the most precise, and imo pretty simple solution, is to use a specified ratio of acid buffers to alkaline buffers to achieve desired KH & pH and then a remineralizing product to increase GH. The products you will want to use depend on if you plan on having a planted tank or not. If you plan on having a planted tank you don't want to use phosphate based buffers and you don't want to use mineral salts that include sodium.
Do you plan on having a planted tank? What kind of fish are you keeping? What is your ideal KH, pH, & GH?