I'm thinking about introducing sambuca (maybe grappa) into my next homebrew. What are the common practices for adding? secondary? bottling? Recommendations for getting the "right" amount without dosing the whole batch?
Put a known volume of beer in tasting glasses or cups. Use measuring spoons to dose each cup with a different amount until you get the level of flavor that you want. Keep notes so that you can do it again. I'm pretty sure that it will scale linearly, so if e.g. an ounce tastes good in 12oz of beer, then around 50oz will taste good in a full (5-gallon) batch. I'd add it to the keg or the bottling bucket, depending on how you intend to serve your beer.
I have always done the mini test being careful to note the volumes and ratio as I build the blend in the pint glass.
When I comes time to execute the whole batch I usually would only add 90-95% of what I calculated from my single glass ratio to the whole batch. Then I would taste it and ease up to the final flavor profile I was looking for.
You can always add more but you can't take it away. Its worked well for me in the past. if you are doing it in a keg its real easy because you can purge with CO2 and mix well, then taste it. If you are doing the addition in a bottling bucket I do it prior to adding my priming sugar because that will screw up the tasting a bit. And I gently mix with a long spoon. Maybe there is a lot of oxidation but its fine for me.
I make a Bourbon Vanilla Imperial Porter that's very popular in the homebrew community. I use both of the techniques described above and it makes it easy to determine the correct amount. I especially agree with having someone else help you with the tasting. I use 2 oz. samples of beer (usually 4 of them) and dose them each with a different measured amount.