Hello Jokerman,
Good advice from Barried, but here is my two penny`s worth. I have not used campden tablets for over 15 years, I use neat powdered sodium metabisulphite. This is available from your home brew shop (if you have one) Campden tablets contain a fair proportion of binder to hold the tablet together, so you cannot be sure how much sulphite you are actually adding. I do not add any initially, only at the end of fermentation. The surest way to add the sulphite is by adding what is called "stock solution" This basically is 2 oz of sulphite added to 1 pint of cold water. When dissolved , add 1 teaspoon of this solution to one gallon of wine, at the end of fermentation & each racking afterwards. BEWARE, THIS IS VERY PUNGENT DO NOT BREATHE IT IN! I usually make up half this amount & keep it in a stoppered bottle, it will keep for many Months. If this does not appeal, just add 1/8th of a teaspoon of neat powder at racking time, this is the equivalent of a camden tablet, but is not as accurate as using the above method.
Hope that helps,
Adrian.