I use ginger in my Christmas Ale at about a rate of 0.5 lbs. per bbl., along with other spices. The ginger is definitely noticeable in the final product with our being over whelming. Brewing this lighter beer, this should be enough, when I use it, I smash up the roots and add them in a muslin bag for the last 10 minutes of the boil.
Brewer Rocky River Brewing