How do I work out my reef's daily two-part dose?
Measure what the tank uses, then replace it. Two-part dosing has one job — hold alkalinity and calcium where they are while the corals pull both out of the water — so the number you need isn't "how much to raise it," it's how much it falls per day. Test alkalinity and calcium, wait three or four days without dosing (or note exactly what you did dose), test again at the same time of day, and divide the drop by the days:
consumption per day = (test 1 − test 2) ÷ days · daily dose = consumption × system liters ÷ product strength
A 100 gallon system that goes from 8.6 to 7.4 dKH and 428 to 419 ppm in 3 days is using 0.4 dKH and 3 ppm of calcium a day. On Balling-strength stock that's 54 mL of Part A and 57 mL of Part B every day; on Randy's Recipe #1 stock, which is stronger, 29 and 32 mL. Behind the liquid that is 4.5 g of baking soda and 4.2 g of calcium chloride a day — which is what a bottled two-part costs several dollars to deliver.
Coral skeleton is calcium carbonate — one calcium ion for every carbonate. In test-kit units that makes 1 dKH of alkalinity ≈ 7.2 ppm of calcium (0.357 meq/L ↔ 20 ppm per meq/L, the figure Holmes-Farley quotes). A healthy, calcifying reef consumes them in that ratio, and a balanced two-part replaces them in that ratio, which is why the two bottles are dosed in equal volumes. The calculator's "balance" line compares your measured calcium drop with what your alkalinity drop predicts: near 1.0 is normal (1.05 in the example above). Well under 1 usually means the calcium test isn't resolving the change (kits read to ±10–20 ppm; over three days that's most of the signal) or something else is adding calcium — kalk, a reactor. Well over 1 means calcium is leaving the water some other way: precipitating on pumps and rock, which is the low-magnesium signature. Either way, don't "fix" the ratio by unbalancing the doses; find out why, and dose alkalinity by its own consumption.
Balling, Randy's recipe, or a bottle — how strong is my two-part?
The calculator needs one thing about your product: how much one milliliter raises one liter of tank. For the classic DIY recipes it derives that from the grams in the recipe. Balling is 1 mol/L sodium bicarbonate (84 g/L, 1,000 meq/L) and 0.5 mol/L calcium chloride dihydrate (73.5 g/L, ~20,000 ppm·L per liter). Randy Holmes-Farley's Recipe #1 is 2¼ cups of baking soda baked to soda ash in a gallon (~1,870 meq/L) and 500 g of calcium chloride dihydrate in a gallon (~36,000 ppm·L per liter) — about 1.9× the alkalinity strength of Balling, so the same tank needs proportionally less. Your own mix: enter grams per liter of stock and the salt. A bottled product: copy the label — "5 mL raises 1 dKH in 25 gallons" — and the calculator turns it into the same strength number. Whatever the strength, the daily dose scales with your true system volume, sump included; that's the number labels can't know.
My alkalinity is low — can I correct it and maintain at the same time?
Yes, and this is the case the one-shot calculators get wrong: a correction poured on top of an unmet daily consumption just gets eaten. Give the calculator a target and a number of days and it adds the deficit, spread evenly, on top of the maintenance dose for those days — then drops back to maintenance. In the example, correcting 7.4 → 8.5 dKH and 419 → 425 ppm over 3 days on Balling stock means 104 mL of Part A and 94 mL of Part B a day for three days (0.37 dKH and 2 ppm of climb per day, comfortably under the 1 dKH and 20 ppm daily ceilings), then 54 and 57 mL a day after that. If the days you pick would push the climb over the ceilings, the calculator says so — stretch the correction, don't shrink the ceiling.
How to set a dosing pump for two-part
Split each day's total into as many small doses as the pump allows — four a day is the minimum, hourly is better — and stagger the two parts so they never enter the water together: Part A on the hour, Part B on the half hour, into strong flow, away from each other's outlet. Big single doses swing pH and alkalinity, and calcium chloride meeting concentrated carbonate at one spot precipitates as chalk on the pump head. The calculator's "per dose" line divides today's numbers by your dose count, and its numbers for the correction days and the days after are separate, because they should be. Then retest in three days: consumption changes as corals grow (up), when the lights change (up), after a big frag-out (down), so a two-part is a dose you revisit, not one you set.
Every two-part on the shelf is a sodium carbonate or bicarbonate solution and a calcium chloride solution — the same salts the alkalinity and calcium calculators dose dry. Bottled products win on convenience and on being pre-balanced; DIY wins on cost by an order of magnitude and on knowing exactly what went in. The calculator treats them identically because chemically they are. What none of them fix is the slow rise in sodium and chloride from months of dosing — that's what water changes are for, and it's the point at which big tanks graduate to a calcium reactor or kalkwasser, which add both ions without the salt.
Magnesium: the optional third test
Enter magnesium on both tests and the calculator shows its drift too. It falls slowly — a few ppm a week — so most reefs top it up monthly with the magnesium calculator rather than daily; if you see it dropping fast, that's the same precipitation story as a calcium balance ratio well over 1, and it's the thing to fix before trusting any of the other numbers.