Best way to add top off if used with kalkwasser is to drip it based on a 24h method. I mix my stuff a day ahead so it can settle. then i take an airline tube and put a knot in it and adjust the drip rate according to how tight the knot is.
Adding freshwater alone can drop you calcium, alk, and even pH levels. But seeing that you have next to nothing worth of corals that will pull calcium out of your water with the exception of coraline alge, you would use less of a dosage.