it depends on what you want your finished product to ferment out to in alch % well a target. use your tripple scale hydrometer and yes add sugar to the sg you want, it should have a % guide along the sg scale. im not familure with using concentrates but i assume you make your juice as directed on the can and add sugar to the must until the desired sg is achieved. i believe you will have a potential of 7 - 8 % without sugar from most (berry, peach, mango, etc) use a finisher or add more sugar after the finished wine is stabelized to increase sweetness of your wine. good luck