I think I may have a simple solution but it is untested and I am new, so I could be wrong. Why not make a solution? Why try to figure all of that out when alcohol is measured by volume? Seems to me like you are making it much more complicated than it needs to be. It's like when they wanted to figure out the volume of the crown jewels and all of the scientists were coming up with these huge mathematical problems until someone just put the crown in a tub to see how much the water raised. If instead of adding sugar you add a mixture of sugar and distilled water, you would only need to treat it the same as if you were proofing neutral spirits. For example if you wanted to make a gallon of 100 proof into 50 proof and wanted to add 2 cups of sugar then put the sugar in a gallon container, add water to fill the gallon container with the sugar already in it, mix it up and the solution add to the liquor.