Grape Concentrate math for finishing wine

[Sparky]

Hi Gang,

I'm trying to determine how many ounces of grape concentrate that I will need to finish a 3 Gallons of lightly oaked Blackberry Port. Port wines are usually around 8-12 brix. I always do a test taste of various degrees of sweetness but I wanted to make sure I had enough concentrate on hand to at least get me in the ball park.
If the grape concentrate is ~68 brix. Let's just say the target brix is 10 degrees. Is there a formula?

If you wanted to chime in about a grape variety concentrate, I'm all ears.

John, Cypress

 

[JasonH]

You can calculate the final brix using this formula....

[brix of concentrate X (amount of concentrate/total amount)] + [brix of wine
X (amount of wine/total amount)] = Final brix

Total amount = the amount of concentrate + the amount of wine