How much (if any) abv will 8oz of 80 proof bourbon add to 5 gall? And I mean 5 gall, hit the mark in the secondary exactly, for the first time! I got 7.44% on app, and 7.3 with Beersmith.
You can do the math, it isn't very hard.
You have 5 gallons of 7.4% abv beer, you add 8 ounces (.0625 gallons) of 40% abv whiskey. Your new total volume will be 5.0625 gallons.
(5 gallons * 7.4% abv + .0625 gallons * 40% abv) / 5.0625 gallons = 7.8% abv