From: Mark McKenna
Subject: Sulfur Formula

So the formula for sulfur additions using potassium metabisulfite (this is what most people just call sulfur - I'm sure it is what you are already using) is as follows:

Desired ppm addition (1 = 100ppm) x Gallons(1 = 1000gallons) x 900 = Grams of Metabi

So for example:
If I had 60 gallons of Zin and I wanted to make a pre bottling add of 20ppm the math would be as follows:

.20 (20 ppm) x .06 (60 gallons) x 900 = 10.8grams

Or if I had a 5 gal carboy that had finished that need an initial post fermentation add:

.40 (40 ppm) x .005 (5 gallons) x 900 = 1.8 grams

As a note on technique, your initial add after finishing malolactic fermentation should be 40 or 50 ppm. During aging it is good to maintain the free SO2 above 25 - 30 ppm. Optimally it should be checked every month. Alternatively, adding 15 ppm every other month and keeping containers full can get you by for a while. Always check and adjust before bottling.