calculating gravity point additions in secondary?

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

I honestly don't know how to do the math on this, but if you have 3 gallons at, say 1.015, and you add 0.375 gal of grape juice or must at 1.075 you'll then have 1.022. In other words six cups of 1.075 adds 8 points to three gallons.

I'm using BeerSmith's dilution tool to figure this out.

Cool, thanks a lot.

what you would do is take a hydrometer or refractory reading of your juiced grapes. that would give you the gravity reading. Then 1 gallon juice would be that specific gravity. times how many gallons you are using, then divided that by the batch size.

So 1 gallon of 1.040 (40) juice in 3 gals 40 / 3 = 13.3 points

Ah OK.
Thanks a lot man.