Thanks for clarifying your question a little bit. I am glad to hear you regularly top off your barrels, it’s a practice all of us need to do. Alcohol and water definitely do evaporate out of barrels (along with small amounts of other volatile aroma constituents of wine) and the resulting headspace does need to get refilled as this occurs. I find that topping monthly, usually replacing between 1⁄4 to 1 gallon every time, is enough to keep my wine sound. The given amount of wine you lose in the “angel’s share” as you refer to it above will change depending on the humidity and temperature of where you store the barrels, as well as the age and stage of your wine. I always lose more volume when my wine is very young and going through ML as it’s so gassy. The CO2 blows off and you lose volume as the wine settles down. And I lose less when the wine is sulfured and simply aging, anywhere after about four months of age. Don’t forget that you also lose volume
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