trying to figure out fringing capacitance expression of an offset coupler

Hello , for the purpose of impedance compensation of step discontinues ,for the expression shown bellow i need the fringing capacitance.
but i only managed to find the odd mode and even mode fringing field capacitance plots of broadside coupler(which is the closest to offset coupler)
how can i get the total fringing field capacitance from having the even and odd capacitance?

Thanks

The step discontinuity structure appears to only support one mode. I don't see how the modal capacitances of the broadside coupler are related.