I do not make kits. but Potassium Metabisulphite is used in wine to preserve and protect it. When added to the wine it does not effect the taste, the resulting SO2 remains in the wine in free form until it binds with O2 once bound it no longer serves as protection. Bottled wine has very little exchange with Oxygen until opened. so there only needs to be enough SO2 to protect it until drank. as wine is racked and Additional O2 is introduced into the wine and So2 is lost through evaporation, precipitation and binding. This is why it is added when racked during bulk aging.
yeasts produce a sulfur-binding compound called acetaldehyde. As this compound is produced through the course of anaerobic fermentation, it is bound up by the available SO2. If you measured the SO2 immediately after fermentation, you would find little to no free SO2 and a total SO2 less than half of your pre-fermentation addition. The resulting loss would be due to binding, precipitation and vaporization. What this means is that for all practical purposes, the winemaker is starting from square one again.
Unless there is a compelling reason, wine destined for malolactic fermentation (ML) should probably not receive any SO2 at this point. Lactic acid bacteria are highly susceptible to SO2. Instead, initiation of ML should take place as soon as possible.
All other wines—whites especially—should receive sulfur at their first racking. The following philosophy makes this addition fairly routine for most wines:
1. I want enough SO2 present to protect the wine.
2. With alcoholic fermentation complete, the wine can be kept cold to retard microbial activity.
3. In most cases the wine won’t be consumed for several months at the earliest, and a slightly elevated SO2 at this stage is not an issue.
4. Better protection is afforded by one larger dose of SO2 than by more frequent smaller doses.
The above philosophy, combined with experience, has led me to a standard addition at first racking of 80 ppm. The dosage of SO2 is calculated and added to the receiving tank as it is filling. I understand a one-size-fits-all answer is simplistic, but an addition at this level usually provides adequate protection through spring without going too high.
Extenuating factors might include plastic tanks, warm cellars, a pH above 3.5, or late-harvest wines, all of which might call for 10-20 ppm more SO2.