You need to account for the K supplied by the KNO3 and KH2PO4 if you want to follow PPS pro exactly. That would approximately halve the required dose of K2SO4. However, that's quite a low dose of K, and extra probably wouldn't hurt. The other problem is you won't be able to dissolve 84 grams of K2SO4 in 500 mL of water (calculator says: The solubility of K2SO4 at room temperature is 120 mg/mL. You should adjust your dose.), but 42g should be fine.