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Sounds right. So a 3 Red and 4 White would make a 7 sparkling. Given that Sparkling only starts at 7, you'll need to make sure your red and white source grapes add up to at least 7.
I would be inclined to say that no matter what grapes get used, it should start at 7 (though this does go against the rules as written). Otherwise, it imposes an unnecessary impediment. Though by the time I’m making sparkling wine I’m making it to fill an order in the current or following year, so its never been an issue.