The Swiss Alcohol Board's "Calco" app enables you to determine the alcohol strength of spirits, as expressed in per cent by volume. You will also need an alcoholometer, which will provide you with the required input information.
With "Calco" you have two functions at your disposal: "Verification" and "Reduction" of spirits.
With the "Verification" function you can calculate in per cent by volume just how much alcohol is contained in the spirit, at the reference temperature of 20°C.
With the "Reduction" function you can calculate how much water you need to add to a spirit in order to obtain the desired consumption strength.