I wish to calculate 13C chemical shift the CH3 carbon in ethyl benzene or similar molecule where the ring current may excert shielding effect on chemical shift of 13C signal.

Would you please suggest me how to include the ring current effect into the DFT calculation using Gaussian. I wish to see how the orientation of the ring affect the chemical shift of 13C, theoretically. The focus is more on the trend rather than the absolute value, so deviation from experimental problem is not the biggest concern.