You bring up a very good question. For the compound you’re talking about, sulfur dioxide, you’ll probably come pretty close to what you would predict based on knowing the volume and the current free SO2. This is just like alcohol, residual sugar, or titratable acidity and can be predicted based on a simple volume to analysis algebraic equation. The one analysis number that never seems to behave this way is pH. This is due to the “buffering capacity” of pH and how it will change based on so many other components in the wine. Because of this, when it’s blended and encounters so many new components, it may respond in slightly unpredictable ways (say, change from a 3.65 to a 3.62, if you were blending two different wines with the same pH together) and will be impossible to calculate as a true algebraic average.
This is where it does get a little tricky with free SO2 and why I say you’ll likely get pretty close . . . and not exact. Free SO2 is pH-dependent so I would expect to get a little FSO2 shift up or down depending on how much the pH of your blend moved. For this reason, it’s always best to re-check your FSO2 after a major move or blend and adjust accordingly.
Want a quickie “cheat” or can’t send your sample to a lab? In my experience moving and blending wine around over the years, you should be safe (or safer) if you add about 10 ppm SO2 to the wine after you’ve moved it. If you’ve come down a bit you’ll bump it back up, and 10 ppm over “ideal” range isn’t a deal breaker. Ideally we measure volatile acidity (VA) and FSO2 every month so we can bump the FSO2 back up when we top monthly. At the very least, you’ll want to measure FSO2 as you approach bottling.