For concentrates made for wine Read directions carefully. Most will tell you directions for a "Full Bodied" Flavor - Normally that means that 128 oz container can be used to make a 3 gallon batch of Full Bodied wine or a 5 gallon batch of wine. ALWAYS go full bodied.
Also Know what you are getting - Is it 100% as Labeled (100% Blackberry Juice) OR is it labeled as Blackberry wine concentrate but the contents reads: Apple Juice, Grape Juice & Blackberry It makes a difference in taste
My standard for drink concentrates (Like you find in the frozen juice concentrates) goes like this. If it's supposed to be made with 4 cans of water, use 3 cans of water. OR a bottle of Concetrated Cherry Juice is supposed to make 1 gallon of juice, I use 4 bottles in a 3 gallon batch.
Bump the concentration UP never down.