Are you sure that pi*r^2 is the volume of a sphere?
I know that pi*r^2 isn't the volume of the sphere but I couldn't remember the formula for the volume of a sphere and regardless, I'm still not sure how to find the percent uncertainty, because my physics teacher isn't very clear when explaining things and it's really difficult to get the just of what he's saying.
Since you say "roughly", there is a rule of thumb that when quantities multiply, their percentage errors add. Since this is a volume problem, it must involve a product of three distances (and since the radius, r, is the only distance involved, radius cubed). That is, the percentage error in the volume is three times the percentage error in the radius.