I've seen a "recipe" somewhere where you mix up the solution and use 1 teaspoon of that per gallon, but I can't find it now. A search on google may help, but I didn't find it with a quick look.
I normally use 1/4 teaspoon per 5 gallons (NOT 1/8 teaspoon per gallon!) for approximately 50 ppm of free SO2. I mix that up in some water, and rack the wine into it. I hope that helps.
I did see this on Jack Keller's site:
Calculation of Sulfur Dioxide (SO2)
Potassium metabisulfite, 1 gram = 150 ppm in 1 gallon, 30 ppm in 5 gallons
Potassium metabisulfite, ¼ teaspoon = 225 ppm in 1 gallon, 45 ppm in 5 gallons
Potassium metabisulfite, 1½ tablespoon to 1 gallon water makes ½% solution for washing equipment
Potassium metabisulfite, 2 oz. to 1 quart water makes 5% solution for sterilizing must (1 teaspoon solution per gallon of must)
1 Campden tablet contains 0.55 grams potassium metabisulfite, yielding 75 ppm SO2 to one gallon of must or wine