Simple:
Don't add any alk, it is high enough already.
For every 1 US gallon of fresh water (RO, or RO/DI, or distilled)
-add 1.1 grams (0.2 tsp) variable from 0.6 to 1.3 grams of Calcium Chloride Dihydrate
-add 5.6 grams (1.3 tsps) of Magnesium Chloride (or the mix with mostly mag chloride, but also some Magnesium Sulfate [Epson salts} for better ionic balance) variable from 3.1 to 6.9 grams
THEN add the salt, let mix for at least a few hours.
Done'sville!
FWIW, the above is what I use for about 1.026 (or so s.g.) with my IO, and have been doing so for the pest 5 or 6 years. This assumes the water starts off at 11, 350, and 1180, and then ends up at 11, 430, and 1380.
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