I cannot drink sweet wines and so I am totally ignorant about dessert wines or ice wines - both very sweet. I would suggest that you bench test this batch. Simply take three or four samples of a known size and add specific and known amounts of sugar to each sample, mix thoroughly and taste. If none are sweet enough then continue the process. And if one glass is not sweet enough and a second is too sweet then the "sweet spot" is between those two and you can bench test between those two.
If 1 lb (say, 400 g)of sugar dissolved in water to make 1 gallon (say 3785 ccs) of syrup will have a gravity of 1.040 then 4 oz in the same volume of water will have a gravity of 10 points and 8 oz will have a gravity of 20 points and I would assume that a dessert wine will have a gravity of 20 or more points... and i so it is simple arithmetic you can use to determine how many grams or oz of sugar you will need to add to the wine to make it as sweet as you would prefer...