Campden = potassium metabisulfite, just in a convenient tablet size. For a 1 gallon batch, you need a miniscule amount of sulfite. It's almost impossible to measure it unless you have a little gram scale.
The easiest way (but a bit wasteful) to do it with small batches and using the powder is to make up a 10% solution of K-meta (10mg in 100ml water), and use 5ml per gallon of this solution.
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