You could set up a little comparison taste test for yourself and see what you like. Most people can begin to detect sweetness in wine in the SG range 1.000 to 1.005. Here is a simple little taste test. Pour four glasses (3 oz. each) of wine. Do nothing to the first, add 1/8 t to the second, 1/4 t to the third and 3/8 t to the fourth. The %-age of sugar for the four samples will be approximately 0%, 0.5%, 1.0% and 1.5%, respectively. Taste each (after all the sugar has dissolved) and see what you like. If one is too dry and the next is too sweet, split the difference in the amount of sugar. Once you find what you like, scale up the amount to the amount of wine you have to sweeten. Be sure to stabilize the wine with K Sorbate before sweetening the entire batch.