Fortified cask wine calculating the angles share?

[Bungbanger]

I'm giving some thought to making a sweet fortified plum wine using slivovitz or some other clear plum brandy. Adding liquor to the wine while half to 3/4 way fermented to arrest yeast for sweetness. I then plan to age in a 10 gallon wine barrel.

I will use an online fortification wine calc for abv. However; is there anyway to figure how much alcohol is lost from the cask while it ages?

Will hydrometers work considering the residual sugars and the liqour being added?

 

[WilliamSlayer]

Pick up a Proof and Tralles hydrometer, it will read high abv liquids.