Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
How fast salinity climbs when top off stops, and how big an ATO reservoir needs to be.
Salt does not evaporate, water does. Every litre lost concentrates what is left. This is why an ATO is the single most useful piece of automation on a reef tank, ahead of dosers and controllers.
S₁ = S₀ × V ÷ (V − evaporated) = 35.00 × 284 ÷ 271 = 36.65 ppt
Evaporation removes water and leaves the salt. Salt mass is conserved, so salinity rises by the ratio of starting volume to remaining volume: S₁ = S₀ × V ÷ (V − lost).
Measure your own rate rather than trusting a rule of thumb. Mark the sump at running level, wait 24 hours with everything on, then measure the RO it takes to get back to the mark. Open tops with fans and a skimmer can hit three percent of volume a day, a covered tank in a humid room under one.
Top off water must be fresh RO or RO/DI, never salt water. Adding salt water to replace evaporation raises salinity twice as fast as doing nothing.
ATO float switches stick. Run two in series, or use an optical sensor with a mechanical backup, and put the reservoir where a failure floods a floor you can mop rather than a floor you cannot.
Size the reservoir for the longest you will ever leave the tank. Ten days of evaporation is a reasonable target, and a reservoir that large also means fewer refills week to week.
Kalkwasser is dosed through the top off line precisely because evaporation is steady and proportional to the tank. That makes the ATO a calcium and alkalinity doser as well, which the Kalkwasser calculator covers.