I posted this on another thread, but felt it should be its own post. I frequently bottle beers with brettanomyces. Brettanomyces will continue to ferment complex sugars and carbohydrates in the beer slowly over time. It's common practice by commercial breweries (Russian River, Crooked Stave, etc.) to bottle these beers when the beer reaches a certain gravity (say, below 1.007), but they will factor in additional gravity points into their bottle priming sugar so that they do not end up with gushers (or bottle bombs), and because they will be aging the beer for 6 months in the bottle before releasing them.

What I'm looking for is a formula for calculating how many volumes of CO2 will be gained if the beer drops 0.001 specific gravity. Or, more simply, how much corn sugar to subtract from my bottle priming to account for a specific gravity change over time in the bottle. I want to subtract a couple of gravity points worth of corn sugar from my bottle priming sugar calculator when bottling my current batch of beer fermented with brettanomyces. Any help with a formula for calculating this would be greatly appreciated. I've found plenty of bottle priming calculators online, but none of them account for additional gravity change over time.

Thanks in advance if you can point me in the right direction.