why not do bench trials with a fixed amount of wine, say 100ml, then add sugar syrup , none the first glass, one to next glass, two to next gals , etc. do taste test and then calculate results for the entire batch. make only 2cups sugar to one cup water for bench trials, then produce what results from calculations for entire batch. Definitely use sorbate and potassium metabisuphite on wine with sugar added. wine will definitely ferment if not done so. wait at least a week after adding sugar before bottling to insure fermentation has not restarted. Use of a hydrometer to add sugar is not a bad guide, but each batch of wine is different and the same hydrometer reading in each batch will not necessarily result in same level of sweetness. batch trials are the best control. also go one past the one batch you think is correct to insure you have the right blend.