When blending helps
If sodium, chloride, sulfate or other minerals are already excessive, adding more salts cannot solve the problem. Dilution reduces concentrations together. Use water suitable for brewing and check residual minerals in RO water.
Calculate the blend
Blended concentration = source fraction × source concentration + dilution fraction × dilution concentration. At 150 mg/L source bicarbonate and an assumed zero in dilution water, 26.7% source water gives roughly 40 mg/L. A 30 L batch of water needs about 8 L source and 22 L dilution water (approximately 2.11 and 5.81 US gal). This is an example, not a universal beer-water recommendation.
Recalculate every mineral
With zero calcium in dilution water, source calcium of 80 mg/L becomes roughly 21 mg/L in that blend. Calculate salts from the mixed profile. Enter measured RO residual minerals when available. A single blend fraction cannot independently match every target ion.
On brew day
Prepare mash and sparge volumes separately. Mix thoroughly and weigh salts for each water volume. Acid may still help: calculate it from the remaining alkalinity. Do not treat the same water twice.