First you need to determine how sweet you want the wine. In my opinion, for a semi sweet wine, a sg of 1.005 is semisweet. sweet is 1.01 ( I like muscadine at that level) and very sweet is 1.015 to say 1.025
Once you know what sweetness you want, you need to determine how much sugar to put in and what the current Specific gravity is. Once you know that, use wine calc to determine how much sugar. for example, if you sg is .998 and you want 1.01, you would need .82 poundds of sugar as determined by using winecalc. Here is a link to download the free winecalc program - very helpful. Do not use decimal points when determining how much sugar. http://mpesgens.home.xs4all.nl/thwp/winecalc.html
Find what volume .82 pounds of sugar is and mix that sugar with the equal volume of water to make a sweetener. Use half or so and taste.
Someone else may have a different idea on the sweetness ratios i gave