Differential signals, even mode impedance?

Hello,

I'm trying to route an analog board that has differential signals. My understanding is that the proper way to do this would be by using coupled microstrip lines with an odd mode impedance of 50, yielding a differential impedance of 100 ohms (what the chip outputs). I'm using the microstrip calculator in from KiCad for a coupled microstrip line and you have to specify even and odd mode impedances. What would be the correct value for the even mode impedance? How does it affect the design?

Thanks for any insight!

cheers
dom