I grew up making sassafras tea as a child, and my Mom made root beer, and we never harvested bark of the tree. We used roots and actually used the bark of the root. If we used the entire root it took much more than just bark. My Dad would create pencil sized slivers in 5 or 10" increments of the entire root (aimed for roots two to four times the diameter of a pencil so he could cut by 2-3-4) and we kept them in the freezer, otherwise we would aim to harvest long, pencil sized roots so my Dad did not have to cut them. He trimmed the root bark off of the large roots, and we dried the remaining root and burned them during bonfires outside.
Unfortunately I do not know what amount of root bark we used, but I know we did use 20" of pencil sized root with bark to.make one gallon of tea. We dug roots any time the ground was soft enough, and quickly discovered when we broke the root line a new tree popped up.