If you have a balance, weigh sassafras 1/.8 = 1.2 times the amount of safrole you need.
If no weighing balance, you need density of sassafras oil, not safrole. then (density/0.8)xvolume = weight of safrole.
If you don't know the density of sassafras, I would guess it's nearly the same as safrole since it's mostly safrole.