I wish I took better notes when I first started out, but I made a vanilla porter and I'm pretty sure I added 2 oz in one 5 gal batch and 4 oz in another 5 gal batch. The low level was too low and you couldn't taste it. You could tell it was there in the batch with 4 oz.
Ideally what you'd do is take a portion of your batch and add some and scale up from that. The problem is the small levels we're dealing with and measuring them at home. The 4 oz in 5 gal is 0.63%, so if you took a 4 oz sample, you'd have to add 0.0248 oz of vanilla to it, which is impossible at home.
Another option is to just add some to the batch, stir gently and taste. Continue until the level is good.
I'm going to run into this in a couple weeks when I try to add maple flavor to a 1 gallon batch. Hope that helps.