There's a very useful sulfite FAQ and calculator here.
How much FSO2 your wine needs is a matter of pH. To quote my friend Daniel,
A wine’s pH affects the free SO2 concentration and must therefore be accounted for when adding sulfite. Specifically, at high pH (low acidity), SO2 effectiveness is greatly reduced and the wine is therefore not as well protected against oxidative effects or microbial organisms which could spoil the wine.
To compensate for the high pH, as a rule of thumb, you should target a free SO2 concentration of approximately 25 percent higher for every 0.1 pH increase above 3.2 (for sweet wines), 3.4 (for white wines) and 3.6 (for red wines). For example, if a free SO2 concentration of 50 mg/L is desired when sulfiting a dry white wine with a pH of 3.8, add sulfite to achieve a free SO2 concentration of 100 mg/L: (3.8-3.4)x10x25%=100% more. Remember to compensate for any free SO2 already present.
Hope that helps out.