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The accuracy with which the "Cooray-Rubinstein" formulation can predict the horizontal electric field generated by lightning return strokes is studied. The horizontal electric field at ground level can be calculated accurately, even for distances as small as 100 m, by using the Cooray-Rubinstein formulation. The initial peak of the horizontal electric field at a height of a few tens of meters as predicted by this approximation may deviate, depending on the conductivity and height, by more than 25% from the exact field at small times. A simple modification that will improve the accuracy to better than about 5% is provided.