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933 g (5 1/4 cups) MgCl2·6H2O + 332 g (2 cups) MgSO4·7H2O to 1 US gal. Stoichiometric 38,116 mg/L Mg and 34,182 mg/L SO4 (no BRS concentration published).
• Part 3 cannot be premixed with the calcium or alkalinity parts (sulfate + Ca -> CaSO4; Mg + carbonate -> precipitate). Some CaSO4 haze in the jug is expected; shake before dosing.
• Ratio choice: sulfate-rich blends (RHF Part 3A, 5:3 by cups = ~1.4:1 by weight; BRS 2-part version ~2.8:1 by weight) are meant to be used WITH a CaCl2-based two-part, whose chloride load needs extra sulfate; a stand-alone Mg supplement for corrections is better at ~10:1 MgCl2:MgSO4 by weight (RHF, relayed; BRS general mix 1285 g:124 g).
• Raise Mg no more than ~100 ppm/day; wait 5 days off-dosing before setting a maintenance dose (BRS).
• Cup measures are approximate; weigh where possible.