I teaspoon per gallon of RO water. Mix it up, let it settle for an hour or so. Drip the solution into your tank overnight. If you have a Nano, all bets are off, but for a 55g - 90g tank, I'd think one gallon overnight is a good starting point. For a 120g - 180g, two gallons would be good.
Of course you must measure the alk, calcium, and pH levels daily to adjust how much to use and take the guesswork out of dripping kalk.
If you want to be extra careful, 1/2 tsp per gallon might be a safer method until you get a feel for how it works.