Carol:
It's easier if you do everything metric and my recommendation is to weigh everything: get away from volumetric recipes. This relies on the fact that in the metric system, for these purposes, 1cc = 1ml = 1gr (close enough; it's really only precisely accurate for distilled water at 20C/68F (?) at sea level).
If the total weight of the ganache (chocolate, dairy, alcohol, other ingredients) is 1000 grams and you've added 10gr of alcohol, then the alcohol is 1% ABV. Simple. 5gr of alcohol in 350gr of ganache? ~1.4%
However, as I think about this, that's not entirely correct, because you need to consider proof. An 86 proof alcoholic beverage is only 43% alcohol. So if you add 10gr of - say - 86 proof rum to a recipe you're only adding 4.3gr of alcohol. The rest is water and flavorings. The math is still easy. In the case of 10gr of rum in a 1000gr recipe, the alcohol content is ~.43% ABV.
Now as I said, this is not 100% accurate past the first digit. But for hobby work, it should be more than close enough. If you plan to sell commercially interstate, then the labeling laws may require more precise measurement.
:: Clay
PS. Take your existing volume measures across making the recipe 5 or 10 times and weigh them. You'd be shocked to find out how inconsistent volume measuring is. Average the weights and use the average as the weight in your recipe.
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@DiscoverChoc