I haven't done this yet, but my hefeweizen is in secondary, I made a orange vanilla extract to add to it. This is what I'm planning to do, according to advice I heard on the Jamil Show. Wait until bottling time, and take a small 2 oz sample out of the bottling bucket. Add a drop of the extract, smell it and taste it. If it needs more, take another sample and add more. If it needs less, take another, larger sample and add a drop. If you figure out, say, that 2 drops per 2 oz is good, then do the math and figure out how many drops for your batch. Mint stout sounds pretty good!