The amount of SO2 added really depends on the pH of the wine. White wines generally have a high pH and therefore require hire amounts to protect from oxidation and browning.
From Presque Isle Wines:
Campden Tablets, 0.55 gram each. 50 tablets. 48% SO2. One tablet per gallon yields about 75 ppm. We recommend 1/2 tablet per gallon at each racking. The tablets need to be crushed before adding. For this reason most people preferred the powdered form (Potassium Metabisulfite).