Make sure it doesn't have any sugar in it. If not, it's safe to add any time. If it might, add it then let it sit a few days to make sure it's all fermented out before bottling.
Not sure if you're asking this, but as far as figuring out how much to use. What I do is, I take a sample using a wine thief. I use a 1 oz shot glass to separate the sample into multiple 1 oz portions of beer. Then I take the stuff I'm wanting to add and use a medicine dropper to add various amounts to the different samples. One drop is very close to 0.05 ml. One ounce is pretty close to 30 ml. You figure out the ratio you want to taste then simply scale it up by multiplication.