Using effective target width (W_e) in Fitts' law has been widely used for evaluating one directional pointing tasks. However, concrete methods of calculating W_e have not been officially unified. This paper concentrates on resolving this problem. A specially designed and controlled experiment is described. The results reveal that the method of mapping all the abscissa data into one unified relative coordinate system to do calculation is better for modeling human computer interfaces than dividing data into two groups according to corresponding target sides and mapping them into two separate coordinate systems.