You can flavor to taste-in the bottling bucket.
add a shot or two with the priming sugar, and rack the beer into the bucket.
Give it a taste and if you need more flavor, add a shot or two and gently stir it in.
Repeat as needed.
As far as how much is too much, I'm not sure.
I'd think with a cup, you should be fine in 5 gallons.
To be safe, add a cup, and bottle half (24 bottles).
Then if you want more flavor, add half a cup to the rest.
(or 4 ozs, bottle two sixes, add 4 more ozs, bottle the remaining two sixes).
Mark the bottles, so if the last ones bottled are flat and taste like vodka, you'll know next time.
-Those that know dont close the door on those that dont know (The Gourds, "Education Song")...
-The brewer's yeast ferments, and forgives, all that you did...(The Gourds, "Right in the Head")...