If it's 100 proof, then it's only 50% ethanol, so 50% water- sucrose has a solubility of between 2 to 4 grams per gram of water, depending on temperature. So conservatively, I'd say half this for solubility in 100 proof alcohol, to get solubility by volume. Sugar is kind of soluble in ethanol, but not like it is in water, so you may be able to get another 1/2 gram or so of sugar in there per gram of alcohol. I was trying to look this up in my CRC book, but haven't found it yet. Anyway, assuming that you're using 100 proof at room temp, adding 2-3 grams of sugar per gram of liquid is going to give you a disgustingly sweet liquid; just for comparison, there's about 14 grams of sugar in a 12 ounce can of soda, which weighs about 300 grams- so we're talking a ratio of roughly .05 grams of sugar per gram of water.
For 190 proof, you're probably talking in the neighborhood of at least .2 to .4 grams of sugar per gram of liquid, which is still about 4 to 8 times more sugar than soda pop.
