For anyone who wants to try grape juice, I use one with 15.6g sugar / 100ml, and I use 50ml of the juice in 500ml bottles, then I top them up with kombucha. That means the total sugar per bottle is 7.6g, you can try using between 5-8g, depending on the amount of carbonation you want.
I find at this sugar level, leaving them for 3 days in 25-28c yields a perfect level of effervescence.
The taste is great, reminds me of sparkling wine.
