I use 1 tsp lemon grass, 1 tsp tarragon, and 1 tsp, lemon zest in my saison that's 6.2%, then carb at about 3.0 vols. Good general rule is to use less than you think, you can always dry hop with a dash more if there's not enough flavor in there for you.
I used some in a Saison. I used two stalks, white part only chopped in the last 10 minutes of the boil. Only a very faint taste. I probably would use slightly more if I did it again.
I agree to err on the side of too little the first time you try it.