One bottle of Rum Flavoring equals 1/4 cup, but I would probably start with a half bottle and add more if desired after cooking a while and tasting. (spendwithpennies.com)
If you use liquid stevia to sweeten the milk, start with just a few drops and add more if desired... it won't take much at all! (naturesknockout.com)
I would start by adding 1/2 teaspoon to the food processor along with the rest of ingredients, then taste the mixture and add more if desired. (golubkakitchen.com)