In my BVIP recipe, I use 375 ml. of bourbon for 5 gal. of beer. It's just enough to taste without screaming "BOURBON!" at you. But I recommend you decide for yourself the same way I did. Pour 4 2 o. samples of the beer with bourbon. Add a measured amount of bourbon to each. Taste them, decide which you like best, then scale that amount of bourbon up for the whole batch.