Sorry if that title was confusing.
The scenario is this:
I've got a quasi-saison going and want to do an experiment with half of it in the secondary, so I'll rack 3 gallons into each of two carboys for the secondary but with the one I'm going to add grape juice (note: homegrown and juiced, not Welch's) oak cubes and pitch some Brett.
Can anyone help me out figuring out how to retroactively calculate the OG with the grape juice? I'll only be adding a quart or 6 cups or so, so maybe it isn't even worth worrying about, but any help would be greatly appreciated.
Thanks a lot