This calculation finds the volume of liquid, of a known concentration, which must be added to a given volume of wine/juice to bring the total concentration up to a desired value:

volume-to-add = (desired-concentration * wine-volume) / (additive-concentration - desired-concentration)

Note that this calculation takes into account the volume of the solution being added, so rather than adding 1L of 100mg/L additive to 100L of wine to give the wine a concentration of 1mg/L, the calculation shows that we need to add 1.010L.