Sure, no reason you can't do that. To get the amount of bourbon I like, I poured 4 4 oz. samples and dosed each with a different amount of bourbon. Determine which you like best and scale up the amount of bourbon to the batch size. Remember, there's no reason to use top shelf bourbon. It shouldn't shout "BOURBON!" at you. You want the bourbon, beer and vanilla to come across as a single integrated flavor.