taste. The higher the the alcohol, the higher the "hotness" (imagine drinkingmoonshine).
I warn you, though, Bo, that this is sort of a daring suggestion.
You've been fermenting 2 weeks, and the SG is currently probably close to 0.990, I can imagine.
Don't worry about it now, but for the next blueberry you make, keep the SG at 1.080 at most and ferment to dryness (SG 0.990). Then sweeten.
The dilution might be a good thing for 1/2 gallon, or you could try the "add a similar wine to top up" method. For that I would choose a nice chardonnay or even a cheapo livingston wine. I usually choose white wines to do my topping up with, but you might want to consider a cab-sav or merlot.
Anyone else have some suggestions?
What's the SG at the moment?