ABV change when adding sugar

[DukeBoxer]

Quick question about mixing up a sweetened drink. How do you figure out the change in ABV when adding sugar to the mix. A simple dilution calculator of alcohol and water is provided here and is easy enough to use, so say I wanted to make a quart sweet tea vodka using 90% alcohol and make it's final ABV be 40%. The recipe on here that I use calls for 1/3 cup of sugar. If I use the dilution calculator to make 30 ounces (up to the neck) of 40% using 90% it says to add 394mL of alcohol and 493mL of water. Also what about using a simple syrup, a 1:1 ratio of water to sugar or even my favorite, agave nectar. I usually sweeten a quart jar with 5 ounces of agave nectar or simple syrup. Who really cares for home use, I know, but what about distilleries or bottling companies. I know they have to re-distill the final product to get an exact amount of alcohol but do they at least have an idea of abv before they go that far while they are in the development stage?