Ian,
It's a very common practice, it's hard to have grapes not ripen in
California but if they needed to bump the sugar up they would use
concentrate. It's illegal for them to use sugar.

As others have mentioned, just think about what this particular
concentrate will do to the flavor. A Concord is very distinctive, while
a Niagara would be less so if you are talking frozen concentrate from a
grocery store. If concentrate from a wine shop, the sky is the limit.


As a rule of thumb concentrate is roughly 70% sugar, but add it in
stages in case yours is different. In other words, if that would work
out to 3 ounces of concentrate add a third of that, taste, etc. There
is nothing more annoying than getting close to the end and making a
mistake that could have been avoided.

Joe