Best advice I can give is to use a hydrometer to help determine the beginning and ending sugar content. I don't think there's a specific formula for sugar, because every different wine base (fruit) has a different amount of sugar. Even between the same fruit... depending on how ripe it is, where it's grown, etc... its sugar content will be different batch to batch. I've got 3 wines going right now... beet, rhubarb and pineapple... and I won't know for a long time, after aging, whether any of these are going to be sweet enough. But if they're not, I'll have to backsweeten to taste. I learned, after trying without one, that the hydrometer is my best friend. They're pretty cheap, and well worth the investment.