Answers will vary depending upon the actual height of the person and the lengths of the shadows. The upper limit will use the maximum lengths for the person and for the lamppost's shadow, and the minimum length for the person's shadow. The lower limit will use the opposites. Assuming that you could know the accurate measure for each of these lengths, the upper limit would be BC = ((AB + 0.25) (EF + 0.25))/(DE - 0.25). (Each of these amounts (AB, EF, DE is the absolute height of the object.)
The lower limit would be BC = ((AB - 0.25) (EF - 0.25))/(DE + 0.25).
For more information on accuracy, go to Session 2, Part C.